Electric Field of a Non-Uniformly Charged Disk

A thin circular disk of radius $R$ has a surface charge density given by $\sigma(r) = \sigma_0 \left(\frac{r}{R}\right)^3$, where $r$ is the distance from the center and $\sigma_0 = 2.0 \times 10^{-6}$ C/m$^2$. Determine the magnitude of the electric field at a point on the disk’s axis, a distance $z = R/2$ from its center. Use the value $\varepsilon_0 = 8.85 \times 10^{-12}$ C$^2$/(N·m$^2$).