By the units of the rate constant, the reaction must be second order:
CO(g) + NO2(g) → CO2(g) + NO(g)
so the general rate law is given by
Rate = k[CO]m[NO2]n
m + n = total order of reaction
Rate = M s–1 = k[ ]m+n
M s–1 = M–1s–1[M]m+n
M = [M]m+n–1
so
m + n – 1 = 1
or
m + n = 2.