A herpetologist is designing a rectangular experimental iguana to fit in the 8ft by 20ft corner of a komodo dragon pen. If only 42 feet of fencing is available for the iguana pen, what dimensions should be used to obtain the largest area for the new fence.

I got the dimensions 20ft by 15ft. I am not sure if that is right. How would all the steps look? I think I skipped one in the end.