Calculate the minimum concentration of Cr3+ (in M) that must be added to 0.073M NaF in order to initiate a pre?

Calculate the minimum concentration of Cr3+ (in M) that must be added to 0.073M NaF in order to initiate a precipitate of chromium (III) fluoride. (For CrF3, Ksp = 6.60 � 10^-11 M4)