You need to make sure there is the same number of each element on each side of the equations. As is, you have:
2 Hydrogen and 2 Oxygen on the left
but only...
2 Hydrogen and 1 Oxygen on the right
The Oxygen is unbalanced. Since you have 2 on the left and only 1 on the right, add a 2 before the H2O on the right. Now, however, the Hydrogen is unbalanced because you have 2 on the left and 4 on the right. Therefore, add a 2 before the H2 on the left. The final equation reads:
2H2+O2=2H2O
Hope this helps!!
