You take c from the co2 when producing ch4 synthetically.

You then reform the co2 on combustion.

Fwiw methane to hydrogen and back again is trivial. It’s how hydrogen is predominantly made today. You can indeed make a hydrogen fuel station from methane. It’s just that it’s really really dumb to do that when hydrogen is so inefficient in an engine and so hard to store. You should just use methane all the way.