How do you find the formula units of a compound?

You need to convert the 650g of Fe to moles first. Then take the 11.6 mol of Fe and use stoichiometry to find the moles of Fe2O3 needed. From the reaction, you can tell that the each mol of Fe2O3 yields 2 mol of Fe. So, to produce 11.6 mol of Fe, you'd need half the Fe2O3 or 5.8 mol. Remember to always work in mol and convert to and from grams using the molar mass.