Question 962462
You're close. You're given that the width of one of the sides is w. You also know that you'll only be fencing three sides, not four, so you want to solve for the length of the rectangle. How can we do this? You know the perimeter, and you know that your fence has three sides. You know the width of the fence, so if we subtract the two widths, we'll be left with the length of the final side!

Thus

length = 150 - 2w

As you probably know, area is length x width for a rectangle. Thus

area = wl = w(150 - 2w)

You were close.