$(ii)$ $2 S + S ^{2-} \rightleftharpoons S _3^{2-} \quad K _2=5.3$
$(iii)$ $S + S _2^{2-} \rightleftharpoons S _3^{2-} \quad K =(?)$
$(iii)$ $=-( i )+$ (ii)
$\therefore K =\frac{ K _2}{ K _1}=\frac{5.3}{1.7}=3.11$
$X:2S{O_2} + {O_2} \rightleftharpoons 2S{O_3}$
$Y:PC{l_5} \rightleftharpoons PC{l_3} + C{l_2}$
$Z:2HI \rightleftharpoons {H_2} + {H_2}$