Textbook Problem

Find the exact volume of the solid that results when the triangular region with vertices at $\left(\mathrm{0,\; 0}\right)$, $\left(\mathrm{5,\; 0}\right)$ and $\left(\mathrm{0,\; 9}\right)$ is rotated about the

a) $x$-axis

b) $y$-axis