do you have to answer in moles, gramms or cubic meters?
If it has to be in volume then you would need the temperature too.
If it has to be in moles or gramms, then it is simple. You calculate the carbon's mass amount into moles: (the mole mass of the carbon is roughly 12gramms/mole), and the methane will be half the amount of moles as the carbon you just calculated, and you can enumerate it to gramms if you have to (M(CH4)=16gramms/mole).
If you have a 95% percentage yield, it means that you converted 95% of the mass of the carbon into methane than the maximum.
So with a 100% yield you would have 0,23 moles (2,75/12) of carbon transform into 0,115 moles of methane, which is 1,84gramms.
Let's say that W(t) stands for the percentage yield, m(k) for the mass of methane you actually got, and m(e) for the maximum mass of metahne you could get from the amount of carbon. In this case W(t)=m(k)/m(e)*100%
So your W(t)=95% in this case, and your m(e)=1,84. From the equation you can calculate that the amount of methane you got is m(k)=1,748gramms. Almost the same as DrunkFace's result, so that will be a good answer imo.
Sorry for the long post but i felt like explain it, rather than just write in a number.