Question

Balance this chemical equation:
Fe(s) + O2(g) ----> FeO(s)

*The 2 is in subscript

Answer 1

See, you have the same initial amount of moles of Fe in reactants and products, so far so good.
Then you see O2 ---> O, there is one O lacking in the product side.
If you mark a 2 in FeO, it becomes 2FeO.
Now you have same amount of Oxygen in both sides but Fe is unbalanced.
1 Fe in reactants and 2 Fe in products, which can be solved by writing a 2 in front of Fe in reactants (2Fe)

2Fe(s) + O2(g) ----> 2FeO(s)