Since you are producing 3.6 mol CO2, you can calculate the starting moles of CH4 with the simple mole-to-mole ratio: 1 mol CH4 / 1 mol CO2 as a conversion factor. Taking 3.6 mol CO2 x 1 mol CH4 / 1 mol CO2 = 3.6 mol CH4 (after canceling out the moles of CO2 on the top and bottom of the calculation)
