Question

V_outside = $\frac{KQ}{r}$
where r is distance of point from the centre of
shell V_inside = $\frac{KQ}{R}$
where 'R' is radius of the shell
$V_B=\frac{K{q}_A}{r_b}+\frac{K{q}_B}{r_b}+\frac{K{q}_c}{r_c}$
$V_B=\frac{1}{4\pi {\in }_0}\left[\frac{\sigma 4\pi a^2}{b}-\frac{\sigma 4\pi b^2}{b}+\frac{\sigma 4\pi c^2}{c}\right]$
$V_B=\frac{\sigma }{{\in }_0}\left[\frac{a^2-b^2}{b}+c\right]$