Answer
$A=x(300-x)=300x-x^2$
Work Step by Step
Step 1. The perimeter of the rectangular fence can be expressed as $2(l+w)=600$, where $l,w$ are the length and width. Letting $x=w$, we have $l=300-x$
Step 2. The area can then be expressed as
$A=lw=x(300-x)=300x-x^2$
