just bring the numbers up and down so there are the same numbers of each element on each side.
on this one i would bring the O2 on the left to 1, bring the right side to 2 so there are 2 oxygens on each side, and because the right side now has 4 hydrogens when you brought up the oxygen, bring the number in front of H2 on the left to 2.
the answer should be 2,1,2
