1-Bromopropane is heated with $\mathrm{AgCN}$ to give $\mathrm{A}$. On hydrolysis with dilute acid, $\mathrm{A}$ gives methanoic acid and an organic compound $\mathrm{B}$. $\mathrm{B}$ is

$\mathrm{AgCN}$ is largely covalent, so attack occurs through nitrogen and the product $\mathrm{A}$ is the isocyanide $\mathrm{CH}{}_3\mathrm{CH}{}_2\mathrm{CH}{}_2-\mathrm{NC}$. Acid hydrolysis of an isocyanide cleaves the C=N carbon off as methanoic acid and leaves the primary amine. $\mathrm{B}$ is propan-1-amine. (KCN would instead give the nitrile and then butanoic acid.)