Find the mass of a ball B given by $\displaystyle x^2+y^2+z^2 \leq a^2$ if the density at any point is proportional to its distance from the z-axis.

I figured that $\displaystyle \rho=Kr$, so the integrand would be $\displaystyle Kr^2$. I think that my limits of integration are wrong. I got $\displaystyle 0\leq\theta\leq2\pi$, $\displaystyle 0\leq r \leq a$, $\displaystyle -\sqrt{a-r^2} \leq z \leq \sqrt{a-r^2} $