A solution contains 0.0018 M Ag+ and 1.6 x 10-5 M Zn2+, and the ions need to be separated. For this purpose CO3^-2 is added to the solution so that both Ag2CO3 (Ksp = 8.1 x 10-12) and ZnCO3 (Ksp = 1.0 x 10-10) can precipitate from the solution. What will the concentration of CO3^-2 be just before the first salt starts to precipitate. (Please give your answer with 2 significant figures.)

You want to calculate the (CO3^-) for each and the lowest is the answer.

............Ag2CO3 --> 2Ag^+ + CO3^2-
Ksp = (Ag^+)^2(CO3^2-)
8.1E-12 = (0.0018)^2(CO3^2-)
Solve for (CO3^2-) needed to ppt the first molecule of Ag2CO3.
Then ZnCO3 --> Zn^2+ + CO3^2-
Ksp = (Zn^2+)(CO3^2-)
1.0E-10 = (1.6E-5)(CO3^2-)
Solve for (CO3^2-) needed to ppt the first molecule of ZnCO3.
If carbonate ion [CO3^2-] is added drop by drop, then the lowest (CO3^2-) shown will ppt either ZnCO3 or Ag2CO3 first.
Post your work if you get stuck.