Should be absurdly easy

this should be totally easy, but I simply don’t understand actionscript beyond a ridiculously rudimentary level (I’m a designer).
All I want to do is a simple calculation. I have 3 input text boxes. Each is a separate variable: width, length, and desired. After the user enters numbers into each text box, I want to hit a button and have Flash perform a calculation and display the answer in a dynamic text box (variable name “total”). Specifically, the calculation would be:

(widthlengthdesired)/7000

Can someone please tell me what code I need to put on the calculate button and what code I need to put on the timeline and anything else I need to make this work.