Answer
$N_2(g)+3H_2(g)\rightarrow 2NH_3(g)$
This is combination.
Work Step by Step
$N_2(g)+H_2(g)\rightarrow NH_3(g)$
This is a combination reaction because two molecules are combining to form a new compound.
The ration of N:H in the output is 1:3, so add a coefficient of 3 to $H_2$.
$N_2(g)+3H_2(g)\rightarrow NH_3(g)$
Add a coefficient of 2 to $NH_3$ to balance $N$ and $H$..
$N_2(g)+3H_2(g)\rightarrow 2NH_3(g)$
