The reaction $\mathrm{CH}{}_3\mathrm{CH}{}_2\mathrm{Br}+\mathrm{N}{}_3{}^{-}\to \mathrm{CH}{}_3\mathrm{CH}{}_2\mathrm{N}{}_3+\mathrm{Br}{}^{-}$ is run at a given temperature. Which changes will increase the rate constant of this SN2 reaction?

Hydrogen-bonding solvents such as methanol suppress an anionic nucleophile, so going from methanol to DMF increases the rate. A crown ether encapsulates the cation and frees the anion from ion pairing, which also increases the rate. Going from DMSO to methanol decreases the rate. Acetate is a poorer nucleophile than azide toward carbon here, so replacing azide by acetate lowers the rate.