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So O2 has zero of heat of formation (or enthalpy of formation). The enthalpy of a reaction is defined like this: H = Hproducts-H reactants. The enthalpy of reaction in this case, is the same as the heat of combustion. Hproducts is obtained summing the heats of formation.
The sign of the heat of formation matters. Using hess's law: 2(-238kJ) + 4(242kJ) + 2(393)= -1279=Heat of reaction Heat of combustion of methanol is just the heat of reaction divided by the molar coefficient for methanol in the chemical equation so about -639.5kJ/mol. In acuality it is somewhere near -727kJ/mol.