What theoretical mass of MgO is produced if 0.150 g Mg3N2 is produced by strongly heating a 2.000 g sample of Mg in air?

2Mg + O2 ==> 2MgO

3Mg + N2 ==> Mg3N2

mols Mg3N2 = grams/molar mass = approx 0.0016 but you need a more accurate answer than that estimate.
Convert mols Mg3N2 to mols Mg used. That's 0.0016 x (3 mols Mg/1 mol Mg3N2) = approx 0.0045.
Convert that to g Mg = mols Mg x atomic mass Mg = approx 0.1
So you must have reacted the remainder of Mg *2.00-0.1 = ?) to form MgO.
Now take g Mg, convert to mols Mg, convert to mols MgO, then to g MgO using the same procedure as I used above.