Since there is only one species on the left to drive the reaction....,
2MgO --> 2Mg + O2
12.09 g MgO (1 mole MgO/40.31 g)(1 mole O2/2 mole MgO)(32 g/1 mole O2)
= 4.80 grams of oxygen gas produced
As you can see from the answer the stoichiometry was not really necessary ( except for instructional purposes ) as simple arithmetic could have given the same answer..