Charge 3 nC is in a hollow cavity inside a large chunk of metal that is electrically neutral. The total charge on the exterior surface of the metal is

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.