This one's pretty easy, and once you get into the discussion of enthalpy a bit more you'll find that it's not too bad.
Enthalpy is the "heat" associated with a reaction, and although we can't measure it directly we can measure the changes. An important thing to remember is that the enthalpy of formation for a pure element in its standard state is zero. This will be very important later on.
To calculate the enthalpy of a reaction, you simply take the sum of the enthalpy of formation of the products and subtract the sum of the enthalpy of formation of the reactants.
(products) - ?Hf
The next thing you'll need is the proper reaction of what you're investigating. Since in your example they want you to determine the oxidation of ammonia based upon the equations they gave you, you simply have to rearrange the three equations into one equation. The reaction you have is almost there, but it's missing the water that's given in equation number 3. I think what you'll need for this question is:
+ 3/2 O2 (g)
? 1/2 H2 (g)
+ NO2 (g)
Now, you just take the ?Hf
s of the products and subtract the ?Hf
s of the reactant; the ammonia. I won't do the math for you, but it should be pretty easy to figure out. Let me know what you come up with.