Answer
(a ) 720000, see table below.
(b) $A=-2x^2+2400x$
(c) $720000$
Work Step by Step
(a )Try values of $x=100,200,300,400,500,600, 700, 800, 900, 1000$, etc. as shown in the table.
The maximum can be estimated to be 720000 when the short side $x$ is 600ft.
(b) Use the figure in the problem, one side of the fence is $x$, the other side would be
$2400-2x$ and the area can be expressed as $A=x(2400-2x)=-2x^2+2400x$
(c) The maximum of the above equation happens when $x=2400/4=600$ which gives
an area of $y=720000$, and this value agrees with the result from (a).