If the rectangle has a fixed width of 300, it means that
[tex]2(l+w)=300 \iff l+w=150 \iff w=150-l[/tex]
where [tex]w[/tex] and [tex]l[/tex] are width and length, respetively.
So given the length [tex]l[/tex], the are would be
[tex]l\times w = l\times (150-l)=-l^2+150l[/tex]