NO2(g) → ½N2(g) + O2(g) DH1 = -33.9 kJ

Question

NO2(g) → NO(g) + ½O2(g) DH2 = 56.5 kJ

What is DH for the reaction ½N2(g) + ½O2(g) → NO(g) ?

Answer 1

Reverse equation 1 and add to equation 2.
Take the negative of DH1 and add to DH2.