College Physics (4th Edition)

(a) The inner surface of the shell has a charge of $-6~\mu C$ (b) The total charge on the outer surface of the shell is $+12~\mu C$.
(a) There is no electric field inside a conductor. Therefore, the inner surface of the shell must have a charge equal in magnitude and opposite in sign to the charge on the sphere. The inner surface of the shell has a charge of $-6~\mu C$ (b) Since the total charge of the shell is $+6~\mu C$, the total charge on the outer surface of the shell is $+12~\mu C$.