The solubility constant or the Ksp is calculated from the product of the equilibrium concentration of the ions involved. For the compound above, the dissociation would be as follows:
MgF2 = Mg2+ + 2F-1
Ksp = [Mg2+][F-1]^2
6.4x10^-9 = x(2x)^2
x = 1.17x10^-3 mol/L
