Flex or Flash For Project?

Hello. I did not have much luck in the AS4 forum for a question I have, but then I found out Flex uses AS3…so I thought I would try here. I am going to try to create a little online app that lets a user create a shape and then determines how many rectangles will fit into that shape. There will only be two widths the rectangles can have, but they can be as long as needed.

The catch is that I have limited experience with Flash and zero experience with Flex. I do have some experience with WPF and C# and now I would like to try making some Flash apps. My question is, how do I approach this learning curve? It seems like Flex would be best suited for my little app, but I don’t know if that is correct.