Given that you are starting with 20.816 grams of Fe and 7.079 grams of O2, what is the THEORETICAL YIELD of Fe2O3 in moles?
This is a limiting reagent (LR) problem; you know that because amounts are given for BOTH reactants.
2Fe + 3O2 ==> Fe2O3
mols Fe = grams/atomic mass = ?
mols O2 = grams/molar mass = ?
Using the coefficients in the balanced equation, convert mols Fe to mols Fe2O3.
Do the same and convert mols O2 to mols Fe2O3.
It is likely that these two values will not agree; the correct value in LR problem is ALWAYS the smaller one and the reagent responsible for that number is called the LR.
The smaller number is the theoretical value in mols.
If you want to convert that to grams (not asked for the in the problem), g = mols x molar mass = ?