since the diameter of the base of the cylinder is 6 feet, then its radius is half that, or 3 feet.
[tex]\bf \textit{volume of a cylinder}\\\\
V=\pi r^2 h~~
\begin{cases}
r=radius\\
h=height\\[-0.5em]
\hrulefill\\
r=3\\
h=9
\end{cases}\implies V=\pi (3)^2(9)\implies V=81\pi[/tex]
