so.. if you notice the picture below
is really just 3/4 of a cylinder, or, a full cylinder, and then you slice 1/4 off of it
notice the right-angle in your picture at the bottom, is cut at a right-angle, meaning, the cutout is 1/4 of the volume
so [tex]\bf \textit{volume of a cylinder}\\\\
V=\pi r^2 h\qquad
\begin{cases}
r=radius\\
h=height
\end{cases}\qquad \frac{3}{4}\textit{ of that}\implies \cfrac{3}{4}\cdot \pi r^2 h[/tex]
