let the metal be M with atmic wt. be x
so 3M + N2 ---->M3N2
      3x                   3x+28
 so,  12/3x   = 14.8/(3x+28)
which in solving for x gives 40
the metal is calcium and its nitride is Ca3N2
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Mass of metal is 12g and Mass of metal nitride is 14.8gm so Mass of nitride ion is 2.8 gm.
By using equivalence law: Mass of metal / Mass of Nitride ion =Equivalent Mass of metal/14 
12/2.8=3x/14
x=20 as x is equivalent weight by multiplying it with 2 which is valency of metal ion. We get 40. 
Before x is 3 is used which is valency of Nitride and 14 is Mass of N .
 
						

							Mass of metal is 12g and Mass of metal nitride is 14.8gm so Mass of nitride ion is 2.8 gm.By using equivalence law: Mass of metal / Mass of Nitride ion =Equivalent Mass of metal/14 12/2.8=3x/14x=20 as x is equivalent weight by multiplying it with 2 which is valency of metal ion. We get 40. Before x is 3 is used which is valency of Nitride and 14 is Mass of N .