You have a rectangle around a rectangle. Since you have
the pool plus deck as having length = (x+25) and width = (x+9)
then x is twice the width of the deck. However, the equation for the total area of pool and deck is still area = length*width
y = (x+25)(x+9)
y = x2 + 9x + 25x + 225
y = x2 + 34x + 225 where x represents twice the width of the deck