Maximum concentration?

Question

a fluoridated water supply contains F- at a concentration of 3.4 ppm. What is the maximum concentration of Mg2+ that can exist in this water supply without causing precipitation? The Ksp for MgF2 is 6.4E-9. answer in grams/L

Answer 1

3.4 ppm means 3.4 mg/L

3.4 / 19 = 0.18 mol/L

6.4 x 10^-9 = [Mg2+]( 0.18)^2

[Mg2+] = 1.98 x 10^-7 M

=> 24.312 g/mol x 1.98 x 10^-7 mol/L =0.0000048 g/L