According to this reaction:
2ZnS(s) + 302 (g) ==> 2ZnO (s) + 2S02(g) AH= - 879 kJ/mol
We know that from 3 moles O2 the reaction releases 879 kJ/mol of energy. We want to know per mole of O2 (1 mole O2)
Procedure:
3 x 1 mole O2 ------ 879 kJ/mol
1 mole O2 ------ X
X = 293 kJ/mol
(notice that "-" means a release of heat)
Answer: 293 kJ/mol