This is what I got hope this makes sense:There is 1 mol of Mg 2+ ions in one mole of MgCl2, we can get that from the formula.
Multiply 3.00 moles by 6.022x10^23 to get moles of MgCl2.
There are 6.022x10^23 ions in one mole of substance.
(3.00)(6.022x10^23)
=1.81x10^24 Mg 2+ ions
Hope it helps though