A solid metal sphere of radius a = 2.5 cm has a net charge Qin = -3 nC (1 nC = 10^{-9)C). The sphere is surrounded by a concentric conducting spherical shell of inner radius b = 6 cm and outer radius c = 9 cm. The shell has a net charge Qout = +2 nC. What is Vo, the electric potential at the center of the metal sphere, given the potential at infinity is zero?