Higher Chemistry Help!

I was hoping that someone could please explain in chemistry calculations like enthalphy of combustion, how do you know what number goes on the top of the fraction,what is on the bottom and what multiplies because i never know which way it is to get the right answer! hope i'v explained it

Here is an interactive on the enthalpy of combustion. Go through it carefully first and then try out the calculations in the interactive test on the second last page.