4 moles of sodium because you originally have 2.0 moles of Na2SO4 and you set that up at the beginning of the (what we called) train tracks. Then diagonally from that, on the bottom you put 1 mol of Na2SO4 to cancel the units. Then above that you have 2 moles of sodium because that is how many there are in the equation that they gave (the Na2 part). Then its just like a fraction, you multiply the top by the top and the bottom by the bottom, which would give you 4moles/1 or just 4 moles Na. I hope that helps
