Verified answer

You need NO on the left, so write equation (1)

(1) NO + O3 ===> NO2 + O2 /\H -198.9 kJ

You need O on the left, so divide equation (3) by 2, write it backwards and reverse the sign of 1/2 /\H

((4) O ===> 1/2 O2 -247.5 kJ

You need to cancel O3 on the left when you add all these equations up, so write equation (2) backwards and reverse the sign odf /\H

(5) 3/2 O2 ===> O3 /\H = +142.3 kJ

Add equations (1), (4), and (5) and add the /\H's

-198.9 -247.5 + 142.3 = -304.1 kJ