Explanation:
In order to maintain electrical neutrality, +3 nC charge which is present in the cavity tends to attract -3 nC charge in the inner surface of cavity.
Since, it is given that -3 nC charge is present in the hollow cavity. So, in order to maintain electrical neutrality -3 nc charge has to be present on the surface of the metal.
Thus, we can conclude that the total charge on the exterior surface of the metal is -3 nC.
