If you start on the left with Fe, there is one of those. Check on the other side and see if there is only one of Fe. Then you see that there is two oxygen on the left. When you look at the right, it is balanced out because of the 2 in front of the H2O. Finally you move over to the 2H2. There is four hydrogen. So look over at the right and see if there is four hydrogen. If there is the exact number of each element on both sides, then it is balanced.
