Let's see
[tex]\\ \rm\Rrightarrow [M_3X_3]\leftrightharpoons {[M^{3+}]\atop 3x}+{[X^{3-}]\atop 3x}[/tex]
So
As solubility and molar mass given
[tex]\\ \rm\Rrightarrow [M_3X_3]=\dfrac{Solubility}{Molar\:mass}[/tex]
[tex]\\ \rm\Rrightarrow x=\dfrac{0.039}{108.64}[/tex]
[tex]\\ \rm\Rrightarrow x=0.00034M=3.4\times 10^{-4}M[/tex]
So
[tex]\\ \rm\Rrightarrow K_{sp}=(3x)(3x)=(3x)^2=9x²[/tex]
So
[tex]\\ \rm\Rrightarrow K_{sp}=9(3.4\times 10^{-4})^2=0.0000010404=104\times 10^{-6}mol^2L^{-2}=104\mu mol^2L^{-2}[/tex]
