Answer
-87.9
Work Step by Step
You are given that there are 3.55g Mg so in order to find the number of moles of Mg you have to divide the mass by the molar mass which is 24.305
So 3.55\24.305 = 0.146 mol.
Then in order to get the heat for 3.55g you need to multiply the number of moles by the total DeltaH and then divide by the number of moles
so 0.146mol X -1204 Kj / 2 = -87.9 Kj