The given reaction is
nBuBr + KCN \(\underset{}{\stackrel{EtOH - H_2O}{\longrightarrow}} \) nBuCN
The reaction is an SN2 reaction. In this reaction, CN- acts as the nucleophile and attacks the carbon atom to which Br is attacked. CN- is an ambident nucleophile and can attack through both C and N. In this case, it attacks through the C-atom.
