Answer
$86.7\text{ yards}\times86.6\text{ yards}$
Work Step by Step
From the graph in part c, the peak point is where $x\approx86.7$.
Solving for $y$:
$$y=\frac{520}{3}-x=\frac{520}{3}-86.7=86.6$$
Thus, the dimensions that yield a maximum area is about:
$$86.7\text{ yards}\times86.6\text{ yards}$$
