How many moles of FeSO4 are present in a 390 mg tablet?
And I got 2.57×10^−3 and it was right

but I didn't get how to do the 2nd part

How many iron(II) ions?
... atoms

You already successfully calculated the number of moles of FeSO4 molecules are present. Since there's only one Fe(II) ion per molecule, that means that there are 2.57x10^-3 moles of Fe(II) as well.

You know the number of moles, so how do you convert that into the number of atoms? Just multiply by 6.02x10^23, right?