Fe + O2 → Fe2O3 is the unbalanced form of chemical reaction.
Balancing the simple molecules first, in this case the O2
Fe + 3O2→ Fe2O3 there are now 6 O atoms on the left, there must also be 6 O atoms on the right.
Fe + 3O2 → 2Fe2O3
Balance the rest of the equation.
4Fe + 3O2→ 2Fe2O3