2H2O2 ------> 2H2O +O2
because we're given one side with 2 hydrogens and 2 oxygens and another sie with 3 oxygens and 2 hydrogens. The hydrogens are balanced but the oxygens are not. You can't make the oxygen a three on the reactants side with a whole number so you multiply by 2.
Now you have 4 hydrogens and 4 oxygens on one side and 4 hydrogens and 4 oxygens on the other :)
