The balanced equation for the combustion of methane (CH4) is CH4 + 2O2 --> CO2 + 2H2O. The equation is balanced because the number of atoms for every element is the same on both the reactant and the product sides.
The equation CH4 + O2 ? CO2 + H2O is not balanced because there are four hydrogen atoms in the reactant side of the equation and only two on the product side. Oxygen is also unbalanced because there are only two oxygen atoms on the reactant side and three on the product side. Carbon is balanced with one atom on each side.