Answer
$Mg_3N_2(s)+4H_2SO_4(aq)\rightarrow $ $3MgSO_4(aq)+(NH_4)_2SO_4(aq)$
Work Step by Step
$Mg_3N_2(s)+H_2SO_4(aq)\rightarrow $ $MgSO_4(aq)+(NH_4)_2SO_4(aq)$
Add a coefficient of 3 to $MgSO_4$ to balance $Mg$.
$Mg_3N_2(s)+H_2SO_4(aq)\rightarrow $ $3MgSO_4(aq)+(NH_4)_2SO_4(aq)$
Add a coefficient of 4 to $H_2SO_4$ to balance $S$.
$Mg_3N_2(s)+4H_2SO_4(aq)\rightarrow $ $3MgSO_4(aq)+(NH_4)_2SO_4(aq)$