You want to build the rectangular dog pen with 20 yards of fencing against your house which is 20 yards wide. which dimensions will give you the most space for your dog?

Ok so your house is 20 yards wide, that akes care of one side
if you have only 20 yards of fencing then hmm
the way to max the space is to make all side equal

20 is for 3 sides
20/3=6.6666666 per side
assuming you don't need to fence the house side

the amount of fencing is 6 and 2/3 by 6 and 2/3 by 6 and 2/3 yards

The correct answer is 6 2/3 hope this helps 

The graph isn’t that clear