Question

I need help understanding how to do the problem

Answer 1

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 $\bf \textit{volume of a cylinder}\\\\ V=\pi r^2 h\qquad \begin{cases} r=radius\\ h=height \end{cases}\qquad \frac{3}{4}\textit{ of that}\implies \cfrac{3}{4}\cdot \pi r^2 h$