Calculating the enthalpy of polymerisation of ethylene given the bond strengths
The mistake I was making was not noticing that every time a double bond is broken, i.e.; completely broken, 2 single bonds are formed in its place.

One bond between the 2 carbons of the same molecule and another between two molecules, the former of which I forgot to consider.

Thus, the enthalpy is in fact $y-2x$