We need to find which has a greater mass, 2.50 x 10^24 atoms of iron or 9.25 moles of magnesium. First, we should convert everything to moles to make the comparison easier. Since one mole by definition is equal to 6.022 x 10^23 units of a substance, we should divide the number of atoms of iron by this number to get the moles of iron.
2.50 x 10^24 atoms iron (1 moles iron/6.022 x 10^23 atoms) = 4.15 mole iron
So we have 4.15 moles of iron and 9.25 moles of magnesium. We can convert each to grams to find the mass of each sample by multiplying by the atomic weight of each element.
4.15 moles iron (55.85 g/mole) = 231.8 g iron
9.25 moles Mg (24.3 g/mole) = 224.8 g Mg
So we can see that the sample of iron has the larger mass of the two.