Last one today, I swear.

A rectangular plot of land that borders on a river is the be fenced along 3 sides using 400 feet of fencing. Express the area of the fenced in land, A, as a function.

I don't even know how to set this one up. I began solving like this:

A=2x+(400-2x)=400

I know that's not even close. I think I need to terms for the width and the length.