What is the molar solubility of tin(II) hydroxide if the Ksp is 3 x 10^-27 ?

            Sn(OH)₂(s) ⇌ Sn²⁺(aq) + 2OH⁻(aq)    Ksp = 3 × 10⁻²⁷
Initial:                             0 M          0 M
Change:                        +s M       +2s M
Eqm:    　　　               s M          2s M

At equilibrium:
Ksp = [Sn²⁺] [OH⁻]²
3 × 10⁻²⁷ = s (2s)²
4s³ = 3 × 10⁻²⁷
s = ³√(3 × 10⁻²⁷ / 4)
s = 9 × 10⁻¹⁰
Solubility of MgF₂ in water = 9 × 10⁻¹⁰ M

Is this second semester general chemistry?  I got an A in this class last semester but I completely forgot how to do anything in this class.