Answer:
+Q
Explanation:
As no electric field can exist (in electrostatic condition) inside a conductor, if we apply Gauss 'Law to a spherical gaussian surface with a radius just a bit larger than the distance of the inner surface to the center (but less tah the distance of the outer surface), the net flux through this surface must be zero, due to E=0 at any point of the gaussian surface.
Therefore, as the net flux must be proportional to the charge enclosed by the surface, it follows that Qenc = 0.
⇒ Qenc = Qc + Qin = -2Q + Qin = 0 ⇒ Qin = +2Q
So, if the net charge of the conductor is + 3Q (which must remain the same due to the conservation of charge principle) and no charge can exist within the conductor (in electrostatic conditions), we have the following equation:
Qnet = Qin + Qou = +3Q ⇒ +2Q + Qou = +3Q
⇒ Qou = +Q
