CH4 + 2O2 --> CO2 + 2H2O
On the reactant side you start with 1 carbon, 4 hydrogen, and 2 oxygen. On the product side you start with 1 carbon, 2 hydrogen, and 3 oxygen. In order to get them equal, you need to put 2 in front of the H2O which equals out the number of Hydrogen on both sides. But Now we must balance the Oxygens. Because of the H2O we now have 4 Oxygens on the product side and only 2 on the reactant. In order to balance this, we put a 2 in front of the O2 giving us 4 hydrogen on both sides, Balancing the equation