Write out the equation first:
CH3COCH3 + 4 O2 —-> 3 CO2 + 3 H2O
Enthalpy of combustion values for carbon and hydrogen correspond to enthalpy of formation of carbon dioxide and water I believe. (Combusting carbon produces carbon dioxide and combusting hydrogen produces water). So you basically are given values of enthalpy of formation for water, carbon dioxide and of course propanone.
This might help you form the relevant Hess cycle now, allowing you to simply just use enthalpy of formation values consistently.