12 gram of an alkaline earth metal gives 14.8grams of its nitride.The atomic mass of the metal is

let the metal be M with atmic wt. be x
so
3M + N₂ ---->M₃N₂
3x ---->3x+28 ; Atomic mass of N=14
so,
12/3x = 14.8/(3x+28)
12/14.8 = 3x/(3x+28)

which in solving for x gives 40,
the metal is calcium(atomic mass=40) and its nitride is Ca₃N₂