(a) Sand meyer's Reaction - (b) The order of basicity is $({CH}_3)_2-{N}>({CH}_3)_3-{N}$ because alkyl group is small, there is no steric hindrance to ${H}$-bonding, So, nature of alkyl group is responsible for basicity of $2^0$ amine. Secondly, there is inductive effect which is important for salvation effect.