Answer
Please see the work below
Work Step by Step
For a reaction of $n $ order,
Unit of rate constant is given by
$$unit_n = (molL^{-1})^{1-n} s^{-1} $$
For a 3rd order reaction,
$$unit_3 = (molL^{-1})^{1-3}s^{-1} $$
$$unit_3 = mol^{-2}L^{2}s^{-1} $$
