= Solution
This labelled part introduces the reversible reaction pair without a separate calculation to perform. For the following derivations, define its forward and reverse <elementary reaction> rates as
$$
r_2=k_2[\mathrm{NO_2}][\mathrm{NO_3}],\qquad r_{-2}=k_{-2}[\mathrm{N_2O_5}],
$$
and the initiating rate as $r_1=k_1[\mathrm{NO_2}][\mathrm{O_3}]$. The <law of mass action> gives $k_2$ bimolecular units and $k_{-2}$ first-order units. \b[Reversibility does not require $r_2=r_{-2}$ while the complete network is reacting]: that equality would impose <chemical equilibrium> on the pair, an extra assumption not needed here.
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