The Ksp refers to the equilibrium constant for a solid dissolving in an aqueous solution.
Ksp= 5.61×10⁻¹¹
The equation for this reaction is:
Mg(OH)₂ (s) ⇔ Mg²⁺ (aq) + 2 OH⁻ (aq)
And the associated equilibrium expression is:
Ksp = [Mg2+] x [OH–]²
Mg(OH)₂ its solid and because of that doesn't have molarity, so its not included.
Ksp = [Mg²⁺] x [OH–]²
5.61x10⁻¹¹ = (x) (0.200)²
x = 1,4025 x 10⁻⁹