The reaction is presented by
C + O2 ==> CO2
The enthalpy of formation of CO2 is –393.5 kJ/mol
We are asked for the enthalpy change of the reaction
Hr = Hf CO2 - (Hf C + Hf O2)
Since C and O2 are elements, their enthalpies of formation is 0
So,
Hr = -393.5 - 0
Hr = –393.5 kJ/mol