The combustion of methane, CH4, releases 890.4 kJ/mol. That is, when one mole of methane is burned, 890.4 kJ are given off to the surroundings. This means that the products have 890.4 kJ less than the reactants. Thus, AH for the reaction - 890.4 kJ.
A negative symbol for AH indicates an exothermic reaction.
CH4 (g) + 2 O2 (g) CO2 (g) + 2 H2O (1)
ΔΗ = - 890.4 kJ