Arthur D. answered 11/15/15
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P=2l+2w
396=2l+2w
divide both sides by 2
198=l+w
198-w=l
A=lw
A=(198-w)(w)
A=198w-w^2
this is a parabola that opens downward and has vertex (h,k) where h="the maximizing width" and k=the area
use -b/2a
-198/-2=99 feet
you want a square 99 feet by 99 feet
A=99*99=9801 square feet
Note: the maximum area will always be a square; therefore all you had to do was 396/4=99 feet
however, depending on what level of math you are in, you probably need the algebra work