Disc method: revolving around x- or y-axis

Let $R$‍ be the region in the first quadrant enclosed by the $x$‍-axis, the $y$‍-axis, the line $y=2$‍, and the curve $y=\sqrt{9-x^2}$‍.

A solid is generated by rotating $R$‍ about the $y$‍-axis.