You want to find the area left over after the pool is built, so subtract the area of the pool from the area of the yard.
Area of Yard= Base x Height = 14x*19x = 266x^2
Area of Circle= Pi x Radius^2 = (6x)^2*pi = 36x^2*pi
Now subtract the two areas:
266x^2-(36^2*pi)
266x^2-36x^2*pi
Take 2x^2 as a common factor:
2x^2(133-18pi)
D: 2x^2(133-18pi)
Hope this helps :)