Zinc hydroxide is a uni-bi type of electrolyte.
For such systems, k sp = 4S^3
where S = solubility in mol/l
Now for Zinc hydroxide, molecular weight = 99.4
so, solubility = 2.1 x 10-4 g/l = 2.11 X 10^-6 mol/l
Finally, k sp = 4S^3 = 4 X (2.11 X 10^-6)^3 = 3.77 X 10^-17