Bond enthalpy help

Struggling with this summary question in my textbook about bond enthalpy:
The difference between the enthalpy change of combustion of successive straight chain alkanes is around -650kJmol^-1. This is because each alkane differs from the previous one by a methyl fragment, -CH_2.
a. Write an equation for the combustion of one of the carbon and two hydrogen atoms in this fragment.
I've done this and my answer is correct, its CH₂+ 1/2O₂→ CO₂+ H₂O.
b. Use bond enthalpies to calculate a value for the enthalpy change of the reaction in part a (including the breaking of the C-C bond).
The answer in the back of the book says -618kJmol^-1, but I've managed to work it out to -1116kJmol^-1and have no idea how you get to -618.

Help will be much appreciated!!!!!

Original post by uselessdoctor

Struggling with this summary question in my textbook about bond enthalpy:
The difference between the enthalpy change of combustion of successive straight chain alkanes is around -650kJmol^-1. This is because each alkane differs from the previous one by a methyl fragment, -CH_2.
a. Write an equation for the combustion of one of the carbon and two hydrogen atoms in this fragment.
I've done this and my answer is correct, its CH₂+ 1/2O₂→ CO₂+ H₂O.
b. Use bond enthalpies to calculate a value for the enthalpy change of the reaction in part a (including the breaking of the C-C bond).
The answer in the back of the book says -618kJmol^-1, but I've managed to work it out to -1116kJmol^-1and have no idea how you get to -618.

Help will be much appreciated!!!!!

Your answer is not correct - the equation you have given is not balanced!.