A rectangular field is to be bounded by a fence on three sides and by a straight stream on the fourth side. Find the dimension of the field with maximum area that can be enclosed with 1000 feet of fence.

~I'm not really sure how to start with this question. I drew a rectangle surrounded by another rectangle, shaded outer side for the stream, but I'm not quite sure what to do next...

If you help me get started, that'd be great :)