Step-1: $2\;NO \begin{array} {c} \overset{k_1}\; \\ \large \rightleftharpoons \\\overset{k_2} \;\end{array} N_2O_2$
Step-2: $N_2O_2 + O_2 \overset{k_3} \rightarrow 2NO_2$
Overall: $ 2NO + O_2 \overset{k} \rightarrow 2NO_2$
$\begin{array}{1 1} k = \large\frac{k_1 k_3}{k_2} \\ k = \large\frac{k_1 k_2}{k_3} \\ k = \large\frac{k_1}{k_2 k_3} \\ k = \large\frac{k_3}{k_1k_2}\end{array}$