Perimeter question, PLEASE HELP!!!!?

A rectangular lot whose perimeter is 320 ft is fenced along three sides. An expensive fencing along the lots length cost $21 per foot. An inexpensive fencing along the two side widths costs only $3 per foot. The total cost of the fencing along the three sides comes to $2385. What are the lots dimensions? A. The longer dimension is ___ feet B. The shorter dimension is ___ feet. Thank you!!