C# How to use a textbox variable

Question

Good day everyone,

 

First of all, thank you for taking the time to read my request, it means alot.

 

I have created a windows form on VB 2019.

 

When a user puts a number into textbox 5, I want the following equation to be calculated and the total displayed in Textbox8.

 

The below works to some level, but if the number is negative (example: -200) it does not work.

 

Also if the user tries to clear the text box, it always crashes with the same error " System.FormatException: 'Input string was not in a correct format.'

 

My guess is this has something to do with the variable I am using and cannot recognise a character.

 

Thanks again people, I really appreciate your time and expertise.

private void textBox5_TextChanged(object sender, EventArgs e)
{
double firstbox;
double answer;
firstbox = double.Parse(textBox5.Text);
answer = firstbox / 1000 * 3.18141 * 2.3 * 1.19;
textBox8.Text = (answer.ToString());
}