Find the Mass of the Surface $S$ Having the Given Mass Density

Short Answer

Find the mass of the surface $S$ having the given mass density.
$S$ is the hemisphere $x ^ { 2 } + y ^ { 2 } + z ^ { 2 } = 9 , z \geq 0$ ; the density at a point $P$ on $S$ is equal to the distance between $P$ and the $x y$ -plane.