Alright, finding the mols of CO2 and then finding the mass of carbon from it. I'm doing it this way:
73cm3 = 0.073 dm3 so 0.073/24 = 0.0030417 mols of CO2.
mass/12+16*2 (the Ar of CO2) = 0.0030417 mols, which means the mass is 32.0365g.
Percentage composition of carbon in CO2 is 12/12+16*2 = 0.272727... and 0.2727 * 32.0365 = 8.737g of carbon. But I can't subtract the mass of carbon from the mass of the aluminium compound, seeing as it's only 0.144g. I tried the way where I didn't convert 73cm3 into dm3, just 73cm3 = 73g but that didn't work either...