Here 3 atoms of the metal combines with 2 atoms of N
Hence, the formula of the compound $={\mathrm{M}}_3{\mathrm{N}}_2$
Suppose the atomic weight of metal = X
28 g of nitrogen combines $=3\mathrm{X}\mathrm{g}$ metal
From question, 28 g nitrogen combines = 72 g metal
$3\mathrm{X}=72$
So $\mathrm{X}=24$
Atomic weight of metal = 24.