Figure $($a$)$ shows a circular disk that is uniformly charged. The central $z$ axis is perpendicular to the disk face, with the origin at the disk. Figure $($b$)$ gives the magnitude of the electric field along that axis in terms of the maximum magnitude $E_m$ at the disk surface. The $z$ axis scale is set by $z_S=20\mathrm{.}0cm.$ What is the radius of the disk?

$0$

$($a$)$

$($b$)$