Surface Integrals, Stokes & Divergence · Challenge problem

Compute the upward flux $\displaystyle\iint_S \mathbf F\cdot d\mathbf S$ of $\mathbf F = (0, 0, z)$ through the horizontal disk $z = 5$, $x^2 + y^2 \le 9$, as a multiple of $\pi$.