A particle of mass ‘m’ is placed at the centre of uniform spherical shell of equal mass and radius ‘a’. Find the gravitational potential at a point ‘P’ at a distance a/2 from the centre?

The gravitational potential at ‘P’ due to particle at centre is   $V_1=\frac{-Gm}{a/2}=\frac{-2Gm}{a};$
The potential due to shell at ‘P’ is  $V_2=\frac{-Gm}{a}$
   The net potential at ‘P’ is,   $V_{net}=V_1+V_2=\frac{-2Gm}{a}-\frac{Gm}{a}=-3Gm/a$