well, the original equation would look like
Pb 2O (Pb 4O) = PbOO+O. This means on the first side, you have 2 Pb and 6 O and on the second side, you have 1 Pb and 2 O. if you put a 2 in front of PbO, you have 2PbO, and now on the second side, you have 2 Pb and 3 O, you now need 3 more O. So you put a 3 in front of the lone O and your balanced equation in Pb2O (Pb4O) = 2PbO + 3O