To balance a chemical equation, the number of atoms of each element must be the same on both sides of the equation. In this case, there are two hydrogen atoms on the left side of the equation, but only one on the right side. There are also two oxygen atoms on the left side, but only one on the right side. To balance the equation, we need to add a coefficient of 2 in front of the oxygen molecule on the right side. This gives us the following balanced equation:

2h2 + o2 -> 2h2o