NaC10 small 3 arrow pointing right NaC1 + 0 small 2

You see you have 3 oxygen on the left, and two on the right, while the rest is OK. You need more oxygen... most suitably 1 more on the right.

One trick, is to get 3 oxygen atoms on the right, from the O_2. You will find that if you put 1/2 in front of O_2, you get 3 oxygen atoms!

Now, let's put it like that:



Now, all is right. If you don't like having fractions, just work it through using your basic math knowledge, that is, multiply throughout by 2, giving:



Now, if you count them up again, the equation will be balanced :)