Output from 2 text boxes is summed in the third

Question

OK, so I have a Windows Forms Application in C++. It has 3 forms. In Form2.h i have 3 textboxes . I want you to give me the command (and where to put it) , so that when I enter values in the 1st and 2nd texbox, they are summed in the 3rd textbox
Here is my code for the Form2.h  ... please edit it and place a right command on the right place, so my idea will work . (P.S. I've read a question simmilar like this here, but it didn't help me).
Code:
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#pragma once
#include <cstdlib>
#include <iostream>

using namespace System;
using namespace System::ComponentModel;
using namespace System::Collections;
using namespace System::Windows::Forms;
using namespace System::Data;
using namespace System::Drawing;
using namespace std;




namespace AreaCalcwithGUIBETA {

    /// <summary>
    /// Summary for Form2
    ///
    /// WARNING: If you change the name of this class, you will need to change the
    ///          'Resource File Name' property for the managed resource compiler tool
    ///          associated with all .resx files this class depends on.  Otherwise,
    ///          the designers will not be able to interact properly with localized
    ///          resources associated with this form.
    /// </summary>
    public ref class Form2 : public System::Windows::Forms::Form
    {
    public:
        Form2(void)
        {
            InitializeComponent();
            //
            //TODO: Add the constructor code here
            //
        }

    protected:
        /// <summary>
        /// Clean up any resources being used.
        /// </summary>
        ~Form2()
        {
            if (components)
            {
                delete components;
            }
        }
    private: System::Windows::Forms::PictureBox^  pictureBox1;
    protected:
    private: System::Windows::Forms::Label^  label1;
    private: System::Windows::Forms::Label^  label2;
    private: System::Windows::Forms::Label^  label3;
    private: System::Windows::Forms::Label^  label4;
    private: System::Windows::Forms::TextBox^  textBox1;
    private: System::Windows::Forms::TextBox^  textBox2;
    private: System::Windows::Forms::TextBox^  textBox3;
    private: System::Windows::Forms::TextBox^  textBox4;
    private: System::Windows::Forms::Label^  label5;
    private: System::Windows::Forms::Label^  label6;
    private: System::Windows::Forms::Button^  button1;

    private:
        /// <summary>
        /// Required designer variable.
        /// </summary>
        System::ComponentModel::Container ^components;

#pragma region Windows Form Designer generated code
        /// <summary>
        /// Required method for Designer support - do not modify
        /// the contents of this method with the code editor.
        /// </summary>
        void InitializeComponent(void)
        {
            System::ComponentModel::ComponentResourceManager^  resources = (gcnew System::ComponentModel::ComponentResourceManager(Form2::typeid));
            this->pictureBox1 = (gcnew System::Windows::Forms::PictureBox());
            this->label1 = (gcnew System::Windows::Forms::Label());
            this->label2 = (gcnew System::Windows::Forms::Label());
            this->label3 = (gcnew System::Windows::Forms::Label());
            this->label4 = (gcnew System::Windows::Forms::Label());
            this->textBox1 = (gcnew System::Windows::Forms::TextBox());
            this->textBox2 = (gcnew System::Windows::Forms::TextBox());
            this->textBox3 = (gcnew System::Windows::Forms::TextBox());
            this->textBox4 = (gcnew System::Windows::Forms::TextBox());
            this->label5 = (gcnew System::Windows::Forms::Label());
            this->label6 = (gcnew System::Windows::Forms::Label());
            this->button1 = (gcnew System::Windows::Forms::Button());
            (cli::safe_cast<System::ComponentModel::ISupportInitialize^  >(this->pictureBox1))->BeginInit();
            this->SuspendLayout();
            //
            // pictureBox1
            //
            this->pictureBox1->Image = (cli::safe_cast<System::Drawing::Image^  >(resources->GetObject(L"pictureBox1.Image")));
            this->pictureBox1->Location = System::Drawing::Point(12, 12);
            this->pictureBox1->Name = L"pictureBox1";
            this->pictureBox1->Size = System::Drawing::Size(204, 111);
            this->pictureBox1->SizeMode = System::Windows::Forms::PictureBoxSizeMode::Zoom;
            this->pictureBox1->TabIndex = 0;
            this->pictureBox1->TabStop = false;
            //
            // label1
            //
            this->label1->AutoSize = true;
            this->label1->Location = System::Drawing::Point(22, 140);
            this->label1->Name = L"label1";
            this->label1->Size = System::Drawing::Size(86, 13);
            this->label1->TabIndex = 1;
            this->label1->Text = L"Value for side a :";
            //
            // label2
            //
            this->label2->AutoSize = true;
            this->label2->Location = System::Drawing::Point(22, 240);
            this->label2->Name = L"label2";
            this->label2->Size = System::Drawing::Size(57, 13);
            this->label2->TabIndex = 2;
            this->label2->Text = L"Perimeter :";
            //
            // label3
            //
            this->label3->AutoSize = true;
            this->label3->Location = System::Drawing::Point(22, 278);
            this->label3->Name = L"label3";
            this->label3->Size = System::Drawing::Size(35, 13);
            this->label3->TabIndex = 3;
            this->label3->Text = L"Area :";
            //
            // label4
            //
            this->label4->AutoSize = true;
            this->label4->Location = System::Drawing::Point(222, 52);
            this->label4->Name = L"label4";
            this->label4->Size = System::Drawing::Size(86, 13);
            this->label4->TabIndex = 4;
            this->label4->Text = L"Value for side b :";
            //
            // textBox1
            //
            this->textBox1->Location = System::Drawing::Point(114, 140);
            this->textBox1->Name = L"textBox1";
            this->textBox1->Size = System::Drawing::Size(86, 20);
            this->textBox1->TabIndex = 5;
            //
            // textBox2
            //
            this->textBox2->Location = System::Drawing::Point(100, 237);
            this->textBox2->Name = L"textBox2";
            this->textBox2->Size = System::Drawing::Size(100, 20);
            this->textBox2->TabIndex = 6;
            //
            // textBox3
            //
            this->textBox3->Location = System::Drawing::Point(79, 275);
            this->textBox3->Name = L"textBox3";
            this->textBox3->Size = System::Drawing::Size(121, 20);
            this->textBox3->TabIndex = 7;
            this->textBox3->TextChanged += gcnew System::EventHandler(this, &Form2::textBox3_TextChanged);
            //
            // textBox4
            //
            this->textBox4->Location = System::Drawing::Point(225, 78);
            this->textBox4->Name = L"textBox4";
            this->textBox4->Size = System::Drawing::Size(100, 20);
            this->textBox4->TabIndex = 8;
            //
            // label5
            //
            this->label5->AutoSize = true;
            this->label5->Font = (gcnew System::Drawing::Font(L"Microsoft Sans Serif", 8.5F, System::Drawing::FontStyle::Regular, System::Drawing::GraphicsUnit::Point,
                static_cast<System::Byte>(204)));
            this->label5->Location = System::Drawing::Point(222, 240);
            this->label5->Name = L"label5";
            this->label5->Size = System::Drawing::Size(68, 15);
            this->label5->TabIndex = 9;
            this->label5->Text = L"L = 2a + 2b";
            //
            // label6
            //
            this->label6->AutoSize = true;
            this->label6->Font = (gcnew System::Drawing::Font(L"Microsoft Sans Serif", 8.5F, System::Drawing::FontStyle::Regular, System::Drawing::GraphicsUnit::Point,
                static_cast<System::Byte>(204)));
            this->label6->Location = System::Drawing::Point(222, 278);
            this->label6->Name = L"label6";
            this->label6->Size = System::Drawing::Size(53, 15);
            this->label6->TabIndex = 10;
            this->label6->Text = L"P = a * b";
            //
            // button1
            //
            this->button1->Location = System::Drawing::Point(349, 280);
            this->button1->Name = L"button1";
            this->button1->Size = System::Drawing::Size(65, 26);
            this->button1->TabIndex = 11;
            this->button1->Text = L"Exit";
            this->button1->UseVisualStyleBackColor = true;
            this->button1->Click += gcnew System::EventHandler(this, &Form2::button1_Click);
            //
            // Form2
            //
            this->AutoScaleDimensions = System::Drawing::SizeF(6, 13);
            this->AutoScaleMode = System::Windows::Forms::AutoScaleMode::Font;
            this->AutoSize = true;
            this->ClientSize = System::Drawing::Size(430, 318);
            this->Controls->Add(this->button1);
            this->Controls->Add(this->label6);
            this->Controls->Add(this->label5);
            this->Controls->Add(this->textBox4);
            this->Controls->Add(this->textBox3);
            this->Controls->Add(this->textBox2);
            this->Controls->Add(this->textBox1);
            this->Controls->Add(this->label4);
            this->Controls->Add(this->label3);
            this->Controls->Add(this->label2);
            this->Controls->Add(this->label1);
            this->Controls->Add(this->pictureBox1);
            this->MaximizeBox = false;
            this->Name = L"Form2";
            this->ShowIcon = false;
            this->Text = L"Rectangle";
            (cli::safe_cast<System::ComponentModel::ISupportInitialize^  >(this->pictureBox1))->EndInit();
            this->ResumeLayout(false);
            this->PerformLayout();

        }
#pragma endregion
    private: System::Void button1_Click(System::Object^  sender, System::EventArgs^  e) {
                 Form2::Close();
             }
private: System::Void textBox3_TextChanged(System::Object^  sender, System::EventArgs^  e) {
         }
};
}
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I've tried something around, but it didn't work ... :( please help me...
THANKS IN ADVANCE.