Question 324005
For a rectangle,
{{{A=L*W}}}
For the fencing,
{{{L+W+W=336}}}
{{{L+2W=336}}}
{{{L=336-2W}}}
Substitute into the area equation,
{{{A=L*W=(336-2W)W}}}
Now area is only a function of x, take the derivative and set it equal to zero.
{{{A=-2W^2+336W}}}
{{{dA/dW=-4W+336=0}}}
{{{4W=336}}}
{{{W=84}}}ft
Then from above, 
{{{L=336-2(84)=168}}}ft
.
.
.
The 168' x 84' rectangle will give you a maximum area of 14,112 sq. ft.