Given the following reactions and enthalpies:

H2O       H2   +   1/2O2                  +643KJ/mole

C2H6    2C   +   3H2                          +190.6 KJ/mole

4CO2   +   6H2O      2C2H6  +  7 O2   +7022 KJ/mole



What is the enthalpy of the following reaction?
CO2      C   +   O2

In this questions you have to rearrange given equations and add them to get the final desired equation.
CO2 ---> C + O2
To get above equation you need to
1)Take equation 2 and divide it by 2  and reverse it so that you get one mole of C ..accordingly - 190.6/2 KJ is the enthalpy of this reaction.
2) Divide equation 3  by 4 ...enthalpy will become 7022/4 KJ
3)Now reverse equation 1 and multiply it by  6/4 ...enthalpy will be -643 *6/4kJ
Add all of them you will get answer.

In  Hess's law questions this is the only way to solve this question