The molar solubility of M(OH)3 in 0.4 M MX3 solution in terms of solubility product of M(OH)3 is
Following equilibria gets simultaneous established
And the solubility of M(OH)3 will get suppressed in presence of 0.4 M MX3 solution.
the solubility of M(OH)3 is suppressed, S' will be negligible. i.e. 0.4 + S' ~ 0.4
Ksp = (S' + 0.4) x (3S')3
⇒ Ksp = 0.4 x S'3 x 27