Explanation:
the volume of a cylinder is as for any other regular 3D object :
ground area ×height
the ground area of a cylinder is a circle, so it is
pi×radius² × height
the radius is always half of the diameter (of the diameter is airways twice the radius).
radius = 6/2 = 3 ft.
the volume is then
pi × 3² × 9 = pi × 9 × 9 = pi × 81 ft³
so, the answer is 81pi ft³.