Which equivalence factor should you use to convert from 3.48 x 1018 atoms of mg to moles of mg?
(1 mol mg/1 atom mg)
(1 mol mg/3.48 x 1018 atoms mg)
(1 mol mg/6.02 x 1023 atoms mg)
(6.02 x 1023 atoms mg/1 mol mg)

The first fact you should know to solve a stoichiometric problem like this one is that, there are 6.02 x 10^23 particles (atoms or molecules) in 1 mole of a substance. 

Now, to know the equivalence factor, it is important to know a technique which cancels like units when they are placed diagonally. Hence, to find the moles Mg from atoms, the equivalence factor must have a numerator with units of moles and a denominator with units of atoms. In this case, atoms would be cancelled, leaving the moles.

The reaction will shift to the right