(a) Sand meyer's Reaction -
(b) The order of basicity is ${\left({\mathit{C}\mathit{H}}_3\right)}_2-\mathit{N}>{\left({\mathit{C}\mathit{H}}_3\right)}_3-\mathit{N}$ because alkyl group is small, there is no steric hindrance to $\mathit{H}$-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.