Question

NH₃ + O_{2 —>} NO₂ + H₂O

Answer 1

$4NH_3+7O_2→4NO_2+6H_2O.$

4 nitrogen atoms and 12 hydrogen atoms are taking part in this reaction. 6 molecules of $H_2O$ . There are 4 molecules of $NH_3$ , there has to be 4 molecules of $NO_2$ as well to balance the number of nitrogen atoms in both side of the reaction.