Question 1021476

It's a quadratic, convert to vertex form.
{{{A(x)=40x-x^2}}}
{{{A(x)=-x^2+40x}}}
{{{A(x)=-(x^2-40x)}}}
{{{A(x)=-(x^2-40x+400)+400}}}
{{{A(x)=-(x-20)^2+400}}}
So the maximum occurs when {{{x=20}}}{{{ft}}} and the value is {{{A(20)=400}}}{{{ft^2}}}