Suppose that $f(x) \ge g(x) \ge 0$. If we rotate the region between $y=f(x)$ and $y=g(x)$, $x=a$, and $x=b$ around the $x$-axis, then each slice becomes a washer with inner radius $g(x)$ and outer radius $f(x)$.
We then have $A(x) = \pi \big(f(x)^2 - g(x)^2\big)$ and our volume is