Find the average potential over a spherical surface of radius R due to a point charge q...

Find the average potential over a spherical surface of radius R due to a point charge q located inside (same as above, in other words, only with z < R). (In this case, of course, Laplace’s equation does not hold within the sphere.) Show that, in general,

where V_center is the potential at the center due to all the external charges, and Q_enc is the total enclosed charge.