struggling with math question

Question

1. Your rectangular backyard is to be fenced with 75 m of fencing on three sides. (see diagram) What dimensions will yield the most area?

Answer 1

Area of a rectangle = A = WL = W(75-2W) = 75W -2W^2

75=L+2W

take the 1st derivative, set = 0, solve for W

A'(W) = 75-4W = 0

4W =75

W =75/4 = 18 3/4 meters wide

L=75-2(75/4) =75/2= 36 1/2 meters long

dimensions that maximize area are 18.75 by 36.5 meters

that's assuming just 3 sides are fenced in. No diagram seems to be shown.