Answer
Length - $600ft$
Width - $1200ft$
Work Step by Step
We have the following values :
Length - $x$ $ft$
Width - $(2400-2x)$ $ft$
Let $A$ be the area of the rectangle.
$A=x(2400-2x)=-2x^2+2400x$
The value of $x$ for the largest possible value of the area is calculated by the formula : $x=-\frac{b}{2a}$
In our case we have :
$a=-2$
$b=2400$
$x=-\frac{2400}{-4}=600$
So, we have :
Length - $600ft$
Width - $2400-2\times600=1200ft$