Here an easy way to see it
1. whip out the periodic table
Fe + O
22. You see that Fe has an oxidation state of 2 and 3. You didn't say if it was Iron [II] or Iron [III] so i'll go with 3 being the most stable one.
Now you have Fe as Fe
+++ 3. See the oxidation state of Oxygen (
O
- -)
Note: when Oxygen is alone, It is always O
2 because its the only way its stable when alone
Now you Have
Fe+++ and
O - - So you switch the charges (Valencies) Fe
2 (the 2 "--" charges from O) and O
3 (the 3 "+++" charges)
So you have
Fe + O
2 --> Fe
2O
34. You got the equation, but now you have a problem
there's one Iron and 2 oxygens on the left and 2 Iron and 3 oxygens on the right so now we have to balance.
5. You can put numbers in front of the Molecules, but now you cant change the subscripts, only the # of molecules.
so
If you put a 2 in Front of the first Fe, a 3 in front of the O
2 molecule both from the left. Then you put a 2 in front of the Fe
2O
3 molecule you get
4Fe + 3O
2 --> 2Fe
2O
3 molecule you get
On the left: 4 Irons 6 oxygens (3molecules of 2 O each)
On the right: 4 Irons and 6 Oxygens (2 mol of 2 Fe and 3

now you have a balanced equation and you are done
You learn a lot when you have a horrible chemestry teacher that doesn't really know what shees doing

So I would like to thank my chemestry teacher for being such a bad teacher, it helped me learn it better myself.

Im not being sarcastic I'm really thankfull
Enjoi and Good night