First, you add the bond enthalpies of all bonds broken in the reactants, then subtract the sum of bond enthalpies of all bonds formed in the products. For combustion, like for methane, you'd consider the C-H and O=O bonds breaking and the C=O and O-H bonds forming. Honestly, I find it easier to remember this when I connect it to my monthly budget—adding all the expenses, then subtracting what I manage to save, to see the final strain. It feels the same kind of tricky calculation, but necessary.
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