well, surely you know what the picture below shows
the perimeter is 2l + 5w
and we know the perimeter is 750[tex]\bf 750=2l+5w\implies \cfrac{750-5w}{2}=l
\\\\\\
A(w)=\left( \cfrac{750-5w}{2} \right)w\implies A(w)= \cfrac{750w-5w^2}{2}[/tex]
now as for the part b) [tex]\bf \cfrac{dA}{dw}=375-5w\implies 0=375-5w\implies 75=w[/tex]
now.. if you run a first-derivative test on the critical point of 75
say, test hmmm 74.99999 and 75.00001
you'll notice the point is a maximum
