Let potential of a disc of radius x is V. For this, see video given below

Now potential energy of this disc with a circular layer of charge$$\begin{gathered} 2\mathrm{\pi xdx\sigma } \mathrm{is} \mathrm{V}2\mathrm{\pi xdx\sigma } \\ \mathrm{put} \mathrm{the} \mathrm{value} \mathrm{of} \mathrm{V} \mathrm{and} \mathrm{integrate} \\ \mathrm{inform} \mathrm{for} \mathrm{any} \mathrm{problem} \end{gathered}$$