How to create callback functions to handle user input from a GUI application in an equation

Hello I have a GUI to calculate a variable called Q, where Q = [1+ sina/(1+b - cosc)]^1/2 . I want the user the enter the values of a,b and c then click a push button calculate so that the push button will display the value in a message box or an edit box called Q. I think I need to use the callback functions but I'm not sure how to do it. Any help is much appreciated.Thank you in advance