there are several ways to get to that value, but they are just different ways to order the calculations you need in order to get there.

The basic idea is that you have the same mass of carbon in CH4 as you do in the CO2 that you make from it. So, how much of 1 tonne (or ton) of methane is made up of carbon (rather than hydrogen). To get that, you need the molecular mass of methane, which is 16 (one C + four H=12+4=16). carbon is 12/16, or 0.75 of methane mass. So, one tonne of CH4 has 0.75 tonne of C.

You then do the same thing, in reverse, to get the mass of CO2. CO2 mass is 12+2*16=44. there are (44/12) tonnes of CO2 per tonne of carbon. However, you only have 0.75 tonne of C in 1 tonne of CH4, so the tonnage of CO2 from 1 tonne of methane is 0.75 (tonne carbon per tonne methane) *44/12 (tonne CO2 per tonne carbon)=2.75 tonne CO2 per tonne methane.

pretty easy, eh?