(2.c) [Pb⁺²] = [I^-] = 2 x 10^-3 M

If [I^-] is twice [Pb⁺²], it must be doubled before substituting it into the Ksp expression.

Remember that when the ions come only from the solid dissolving, the concentrations are related . Thus [I^-] = 2[Pb⁺²] = 2x