Given the following data:
2o3 -> 3o2 (enthalpy = -427 kj)
o2 -> 2o (enthalpy = +495 kj)
no + o3 -> no2 + o2 (enthalpy = -199 kj)

calculate the enthalpy for the reaction
no + o -> no2 using hess's law.

Just try to find how you will get the reaction by adding the other reactions. Reverse the second eq as O is in reactant side and half it..

Now in the final reaction there is neither ozone nor oxygen so now you have oxygen on the products side after adding reversed

 2 and 3, O ->1/2 O2 

adding this to the third eq you get, 

NO+O +O3 -> NO2 +3/2 O2

 Now you need to remove O3 and O2 using the first equation, 

So reverse the first one,half it and add, 3/2O2-> O3 adding it removes O3 and O2 you get the requires equation,

 NO + O->NO2 you can do the calculation bit yourself when you add reactions Enthalpy is also added by Hess law so yeah that part should be pretty simple.

the answer is c.the valence of the atoms combine .