Calculate the energy needed to sublime a 1 mole of solid magnesium?

You left out one piece of necessary information, IE₁ of Mg = 738 kJ/mol (*converted to kJ/mol from link below). The strategy to use here is to write a thermodynamic cycle (a Born-Haber cycle) and to isolate the information that is missing. When you do that, the sublimation energy is 153 kJ/mol; the cycle is shown in the image in the second link below. Adding all the quantities shown in the cycle up gives the ∆H_f of MgCl₂. The .png file below shows that - then it is solved for the unknown energy of sublimation (S_M).