Given the following reactions and subsequent ∆H values what is the ∆H for the reaction below?

4CO2 (g) + 2H2O (g) 2C2H2 (g) + 5O2 (g)

You do these problems by finding the heat of formation of each compound, from a table.

Then the heat of reaction is the (sum of the heats of formation of the products) minus (sum of the heats of formation of the reactants).

In your case, the ∆Hrxn = 5∆Hf (O2 (g)) + 2∆Hf (C2H2 (g)) - 2∆Hf (H2O (g)) - 4∆Hf (CO2 (g))