Optimize area of side-by-side rectangular plots

This is a little applet that lets you change the length of the one side to see how the area varies in the rectangular-bug-plots-area optimization problem we did in class 4/23.
Drag the X slider around and see what happens to the area of the rectangle. What X gives you the biggest area? What's the biggest possible area? Does your answer match what we did in class? What happens at the endpoints? (X certainly can't get smaller than 0 or bigger than 30.)