NH3 + O2 -> NO + H2O
On your left are your reactants; NH3 + O2
On your right are your products; NO + H2O
Make a T - chart, writing reactants and products
On your left side write your products down;
N: 1
H: 3
O:2
On your right side write your products down;
N: 1
O: 2 (one on NO and the other on H2O
H: 2
Then you compare both sides to make them both equivalent or equal.
On your left your reactants should look like;
N: 1 x 2 = 2
H: 3 x 2 = 6
On your rights your products should look like;
N: 1 x 2 = 2
H: 2 x 3 = 6
Notice how I did not write O2. That’s because on your left, your reactants won’t match.
It will look like this;
On your left;
O: 2 x 5/1 = 10/2 = 5/2
On your right;
O: 2 + 3 = 5
You can’t have a fraction in your balances equation so the next thing you’ll do is multiply everything by 2.
2 •(2NH3 + 5/2O2 -> 2NO + 3H2O)
Finally your answer will be;
4NH3 + 5O2 -> 4NO + 6H2O
Hope this helps!