Say we have a spherical shell of outer radius b and inner radius a. The shell has a total charge +3q and at it's center is a point charge of charge -q. I know that the E field for r>b would simply be: E = (3q-q)/(4πr^2ε0) and thus the electric potential inside the shell must be the same as the electric potential on the outer shell since there is no E field inside the shell. This brings me to the conclusion that the electric potential inside the shell is simply 2q/(4π(b^2)ε0). This confuses me however as can't it be said that if we take the electric potential at A with respect to the point charge, we get -q/(4π(a^2)ε0)? How can this be the case at the inner surface of the shell if the shell itself has an electric potential of 2q/(4π(b^2)ε0)? Am I looking at this wrong as in one case I'm comparing to infinity and in the other I'm comparing with the point charge? If this is the case does this mean the electric potential at the center of the sphere would simply be 2q/(4π(b^2)ε0) + -q/(4π(a^2)ε0)?