Regents Algebra I Practice Test 3

Petra has 60 feet of fencing to enclose a rectangular garden beside a barn. The barn wall forms one side of the garden. The side opposite the barn includes an existing 20-foot hedge, so fencing is needed only for the two sides that meet the barn and for the rest of the side opposite the barn.

Let $x$ be the length, in feet, of each side that meets the barn. What is the maximum area, in square feet, that Petra can enclose?