You have to figure out the area of the backyard first by finding it's dimensions. We know that 2 of the sides are 80ft. Thus the total length of the other two sides are 210ft since 370ft - 160ft = 210ft. Obviously the length of the other two sides would be half this length since they are the same length and would be 105ft.
The area of the backyard is 80ft X 105ft = 8,400ft^2
The area of the pool is 20ft X 35ft = 700ft^2
Area of backyard without pool is 8,400ft^2 - 700ft^2 = 7,700ft^2