The stoichiometry and solubility product of a salt with the solubility curve given below is, respectively:

From the given curve,

if $[X]=1 \mathrm{mM}$ then $[Y]=2 \mathrm{mM}$

$\therefore \quad$ Salt is $\mathrm{XY}_{2}$

$\mathrm{K}_{\mathrm{sp}}=[\mathrm{X}][\mathrm{Y}]^{2}=\left(10^{-3}\right)\left(2 \times 10^{-3}\right)^{2}=4 \times 10^{-9} \mathrm{M}^{3}$