Answer 1

Let us first work out the valency of the metal M (we know that carbonate ion has a valency of −2):

Element/Ion  M  `"CO"_3^(2-)`

It is clear that M has a valency of 1.

Formula of nitride of M:

Element / Ion   M   N^3-
  Valency           1   -3

Crossing the valencies, formula of the nitride of M is M₃N.