CH3OH + O2 = CH2O + H2O
If you add number 2 before some of these elements, they will be balanced. So:
2CH3OH + O2 = 2CH2O + 2H2O
On the left, there are 2 atoms of C, 4 atoms of O (2O + O2), and 8 atoms of H (2 * H3 + 2H = 8H), and on the left, the same number of atoms is present.
