What is the solubility, in $mo\frac{l}{L},$ of $PbC{l}_2$ in a $0\mathrm{.}65mo\frac{l}{L}$ solution of NaCl, if the $K_{ap}$ of $PbC{l}_2$ is $3\mathrm{.}6{10}^{-9}?[4$ Marks$][$T$/$T$]$