A rectangular animal pen is to be built against a straight section of river, so only three sides require fencing. The side parallel to the river costs \18perfoot,whileeachofthetwosidesperpendiculartotherivercosts10 per foot. If the farmer can spend at most \4800onfencing,letx$ be the length (in feet) of the side parallel to the river. Determine the dimensions of the pen that maximize the enclosed area, and state that maximum area.