Answer is: Ksp = 4s³.
Balanced chemical reaction (dissociation) of strontium hydroxide:
Sr(OH)₂(s) → Sr²⁺(aq) + 2OH⁻(aq).
Ksp(Sr(OH)₂) = [Sr²⁺]·[OH⁻]².
[Sr²⁺] = s.
[OH⁻] = [Sr²⁺] = 2s
Ksp = (2s)² · x = 4s³.
Ksp is the solubility product constant for
a solid substance dissolving in an aqueous solution.
[Sr²⁺]
is equilibrium concentration of iumcations.
[OH⁻] is equilibrium concentration of hydroxide anions.