where the solubility product Ksp is applied where salts don't fully dissolve in a solvent.
and when the reaction equation is:
AB2(s) ↔ A2+(Aq) + 2 B-(aq)
so we have the Ksp expression = [A2+][B-]^2
when we assume [A2+] = X = 0.01 M
and [B-] = 2X = 2*0.01 M = 0.02 M
So by substitution:
Ksp = 0.01 * (0.02)^2
= 4 x 10^-6