Calculate the enthalpy of the following reaction:

2Mg + O2 > 2MgO

Given:

Mg + 2HCl > MgCl2 + H2. H=-21.0 kJ

MgO + 2HCl > MgCl2 + H2O. H=-33.5 kJ

H2 + 1/2 O2 > H2O. H=-76.0 kJ