If the K_SP  of AgCl is 8 × 10^-12, the molar solubility of AgCl in 0.1M AgNO₃ solution is

$\mathrm{AgCl}\rightleftharpoons {\mathrm{Ag}}^{+}+{\mathrm{Cl}}^{-}$
(S+0.1) S
$\begin{array}{l}{\mathrm{AgNO}}_3 \to {\mathrm{Ag}}^{+}+{\mathrm{NO}}_3^{-}\\ 0.1\mathrm{M} 0.10.1\end{array}$
S<<0.1 hence “S” can be neglected.
${\mathrm{K}}_{\mathrm{SP}}=0.1\left(\mathrm{S}\right)\mathrm{S}=\frac{K_{SP}}{0.1}=\frac{8\times {10}^{-12}}{{10}^{-1}}=8\times {10}^{-11}\mathrm{M}$