To calculate the number of moles, the equation is mass of substance divided by the molar mass of substance.
Molar mass of Fe2O3 is the relative atomic mas of all the elements from the substance added up, which makes 55.845x2 + 16.0 x3 = 159.69g mol-1
So the number of moles is
545g / 159.69gmol-1
=3.41 mol (corrected to 3 sig. fig.)
