The enthalpy change for the overall process is simply the sum of the enthalpy changes for thse two steps. You will see that the gaseous water appears on both sides and so cancels out. The overall enthalpy change is the same regardless of the number of step taken to achieve it. This is analogous to travelling from Ames to Des Moines but going through Colorado, California, Texas, Virginia, New York and so on. It is still only about 30 miles from Ames to Des Moines.
Hess law is great fro determining enthalpies that are difficult to determine directly.
The problem here is that O is unstable and would not be available to prepare a reaction. You can very quickly become overwhelmed by this type of problem. Even for an experienced scientist, a certain amount of thinking time is required to re - orient the equations and balance them to obtain the correct proportions. Look to find reactants and products in the given equations. Here we are looking for NO and O as reactanst and NO 2 as products.