Newb needing help with as

I am an artist. So the designer aspects of flash are second nature to me, but the developing side of flash is very foriegn. I have a project that is simple enough in theory, and on paper, yet I cant seem to get anywhere when I try to write AS in flash 8 pro. I am a newb at AS… so please be easy on me as I am trying to learn…

on with my request…

I build signs for a living and I want to create an form that will calculate the cost of materials based on an initial size of the sign. I am at step one with the scripting after having laid out some input text fields for width, height, and area.

the catch is this… I need the area text field (named A_txt) to calculate the width input text field (named W_txt) and height input text field (H_txt) and auto update A_txt as the numbers are inputed on the user side. The area text field will be the initial number for other calculations later in the form for determining the price of the customers sign.

I think that “onChanged” is what I should be using but I am frustrated and confused because I dont know how to write math equations with AS. My knowledge is only advanced far enough to tellTarget(gotoAndPlay)… stop laughing =P… any help or links would be very much appreciated