$$\begin{gathered} We know that \\ \sum _{plane}EdS\cos \theta = \frac{q}{6{\epsilon }_0} \left(1\right) \\ and Net force on the plane =\sum _{plane}{F}_{\perp }dQ \\ =\sum _{plane} qE\cos \theta dS\sigma =\frac{Q}{d^2}\sum _{plane} qE\cos \theta dS \left(2\right) \\ Now solve eq. \left(1\right)\hspace{0.17em}and \left(2\right) \end{gathered}$$