How do I balance the chemical equation: C2H4 + O2 --> CO2 +H2O

First take a look at the equation and see how many elements there are, in this case we have 3 different elements; Carbon, Hydrogen and Oxygen. To balance the equation start off by balancing the elements in the complex molecules first and then the single element molecules after. We'll start with balancing the Carbons. On the left we have 2, but on the right only 1, so we multiply the CO2 on the right by 2. Next we look at the Hydrogen. We have 4 on the left, but only 2 on the right. To balance them, we multiple the H2O by 2. 

Now we have to take a look at the Oxygen. From balancing the Hydrogen and Carbon, we have changed the initial equation. No looking at the Oxygen on the right-hand side, we have 4 from the CO2 and 2 from the H2O for a total of 6. In order to have 6 Oxygen on the left as well we have to multiply O2 by 3. Now we have the balanced chemical equation of: C2H4 + 3O2 --> 2CO2 + 2H2O