\b[A is dimethyl <ether>, $\mathrm{CH_3OCH_3}$], with NaCl as the corresponding salt byproduct. Backside methoxide attack on methyl chloride gives an <SN2> substitution:
$$
v=k[\mathrm{CH_3Cl}][\mathrm{MeO^-}].
$$
The rate is bimolecular because both reactants participate in the elementary bond-forming/bond-breaking event.

Solution

Past exam of the chemistry course of the year 1 of the natural sciences course of the University of Cambridge / 2021 / Paper 1 / 3 / a / i / Solution

 Solution (source code)