Very Interesting Event handling Challenge

Question

1. Custom Control with 2 Textboxes 
Publicly define Int Sum = textbox1 + textbox2 values 
2. A new Form and Add the Custrom Control created 
A third Textbox which will take value from CustromControl.Sum 

The Tricky issue is , Event Should Occur when ever the Textbox1 and Textbox2 values are changed in the Form . and Textbox 3 should get automatically updated. 

can anyone who's done this help me with this ?? Its something I've tried using A delegates and events but its never getting invoked in the main form . 

Any Help would be appreciated 

Thanks