urgent enthalpy of formation and combustion

[befuzzled!!]

i'm doing AS chemistry btu last lesson teacher was off and so was given the work with homework to do for next lesson but i have no idea what i'm doing
i know the meaning of them but i don't know how tto calculate them :S
please help urgently if anyones awake

[north star]

if you';r stuck in the snow and didn't get to school today then have a look at this

http://en.wikipedia.org/wiki/Standard_enthalpy _change_of_formation

Hess' Law is perhaps the most important point.