Enter the the ksp expression for the solid ab2 in terms of the molar solubility x.

Ksp is the equilibrium expression for the dissolving of a solid. The solid ab2 dissolved with the formula ab2(s)-->a(aq)+2b(aq) so the equilibrium comstant is K=([a][b]^2)/[ab2] and since solids are not put into the equation due to the fact that solids don't really have a concentration, it simplifies to K=[a][b]^2. Since Ksp is is how much solute can dissolve at equilibrium Ksp=[a][b]^2. Molar solubility is just another way of saying molarity so if you make x=a you know that b=2x since there is twice the amount of b than a. With those values for a and b in terms of x you can make Ksp=[x][2x]^2 which means that Ksp=4x^3. I hope that helps. Let me know if anything is unclear.