Maybe you didn't see but it's 9 ft, not yard.
1 ft = 0,333 yd
9 ft = 3 yd
Ok we'll start by calculate the triangle
Because of the 90°, you will use 3yd for the height and 4yd for the base
(3•4)/2 = 6 square yards
For the third dimension,
6•7=42 cubic yards
Total volume = 42 cubic yards
