Answer is: molar solubility of barium carbonate is 4.47·10⁻⁵ M.
Chemical reaction:BaCO₃(s) → Ba²⁺(aq) + CO₃²⁻(aq).
Ksp = 2.0·10⁻⁹.
s(BaCO₃) = ?M.
s(BaCO₃) = s(Ba²⁺) =s(CO₃²⁻) = x.
Ksp = s(Ba²⁺) · s(CO₃²⁻).
Ksp = x · x.
2.0·10⁻⁹ = x².
x = √2.0·10⁻⁹ = 4.47·10⁻⁵ M.