The solubility product of AgCI is $2.6\times {10}^{-10}$ at ${25}^{0}C$.  The solubility of NaCl in 0.2 M  $AgN{O}_3$ solution is 

$AgCI\left(s\right)\overrightarrow{\leftarrow }A{g}^{+}\left(aq\right)+C{I}^{-}\left(aq\right)$

                         (s+0.2)       s

$2.6\times {10}^{-10}=(s+0.2)s$

$2.6\times {10}^{-10}=s^2+0.2s$

$2.6\times {10}^{-10}=0.2s$

($s^2$ is negligible when compared with ‘s’)

$s=\frac{2.6\times {10}^{-10}}{0.2}$$=1.3\times {10}^{-9}\mathrm{mol}{\mathrm{L}}^{-1}$