Answer
$KSP $= $[A^{4+}{}_{(aq)} ][B^{2-}{}_{(aq)}]^{2}$
Work Step by Step
Since we know the charge of B and the overall product, we can write the decomposition equation as shown below:
$AB_{2}{}_{(s)} --> A^{4+}{}_{(aq)} + 2B^{2-}{}_{(aq)}$
Now to write out solubility product constant expression, we don't include solids and only include the concentration of the aqueous solutions in terms of Products/Reactants where the coefficients in front of the species become exponents for that species. Therefore, the Ksp expression is shown below:
$KSP $= $[A^{4+}{}_{(aq)} ][B^{2-}{}_{(aq)}]^{2}$
