In this case we have a molecule to find out what is the mass of it, first we define what is the molar mass of this molecule, which means, how many grams there are in 1 mol of Fe2O3, we can do that by summing all the atomic masses in the molecule:
Fe = 55.84 grams, since we have 2 Fe = 111.68 grams
O = 16 grams, since we have 3 O = 48 grams
Now we add them together = 111.68 + 48 = 159.68 g/mol, this is the molar mass of Fe2O3, now let's find out what is the mass when we have 0.75 moles
159.68 g = 1 mol
x grams = 0.75 moles
x = 119.76 grams of Fe2O3 in 0.75 moles