Calculating a number?

[John Smiley]

I previously posted how I am a newbie and got tremendous response from the list. After having explored a bit and tinkered with the program, I have a question about how to do something.

I am wanting to create an application where the user enters data into several fields and the software calculates the answer. In the final product the user will enter 5 numbers and select three numbers from three different drop down menus. The software will calculate a number based on this input and compare the result to 8 other variables the user selects and come up with a "yes" or "no" result.

Rather than trying to find out how to do all of that in one question, I think it best if I learn first how to write the code to calculate (either add or multiply) a number between two fields and have the result output into a third field first. 

Anyone care to explain?

Thanks -

John Smiley
New Orleans, LA