Explanation

(a)
Silver metal is extracted by cyanide process.
${Ag}_{2}S + 4NaCN \rightleftharpoons \ 2Na\lbrack Ag(CN)_{2}\rbrack + {Na}_{2}S$
Argentite sodium argentocyanide
$2Na\lbrack Ag(CN)_{2}\rbrack + Zn\ \longrightarrow {Na}_{2}\lbrack Zn(CN)_{4}\rbrack + 2Ag \downarrow$
Sodium tetracyano ppt.
Zincate (II)