A Solid, Conducting Sphere of Radius a Carries an Excess $\mu$

A solid, conducting sphere of radius a carries an excess charge of +6 µC. This sphere is located at the center of a hollow, conducting sphere with an inner radius of b and an outer radius of c as shown. The hollow sphere also carries a total excess charge of +6 µC.
-Determine the excess charge on the outer surface of the outer sphere (a distance c from the center of the system) .

A) zero coulombs
B) -6 $\mu$ C
C) +6 $\mu$ C
D) +12 $\mu$ C
E) -12 $\mu$ C