Chemistry
posted by Lucy .
Calculate the enthalpy change for the reaction 2C + H2 yield C2H2 given the following reactions and their respective enthalpy changes:
C2H2 + 5/2 O2 yield 2CO + H2O 1299.6
C + O yield CO2 393.5
H2 + 1/2 O2 yield H2O 285.9
I don't even know how to start and the example in the book doesn't explain anything!!!!

Look in your text about Hess' Law. That is what this problem is. The idea is to multiply (if necessary) an equation AND/OR reverse it so that all of them added together will give you the final equation you want.
If you get stuck let us know what you don't understand (be specific; i.e., I don't know where to start or I don't have a clue doesn't help us determine the problem you are having.) 
Check you post carefully. I don't think all of the equations you have written are balanced. For example, look at the ones with CO. Are those CO2?