Calculate DH for the reaction?

Question

Given the following data:
2O3 --> 3O2 DH = -427 kJ
O2 -->2O DH = +495 kJ
NO + O3 --> NO2 + O2 DH = -199 kJ


Calculate DH for the reaction
NO + O --> NO2

Answer 1

It's the third, minus half the second, minus half the first.