For a sparingly soluble salt of molecular formula $A_{2} X_{3}$, if solubility in pure water is $y M$ at a given temperature,
$A_{2} X_{3} \rightleftharpoons 2 A^{3+}+3 X^{2-}$
$2_{y} M\,\, 3 y M$
Solubility product, $K_{s p}=\left[A^{3+}\right]^{2}\left[X^{2-}\right]^{3}=(2 y)^{2}+(3 y)^{3}$ $=4 y^{2}+27 y^{3}=108 y^{5} M^{5}$