Hello, I need help with a solubility problem, I am not looking for the answer just how I should go about solving.

Solubility product of CaCO3 at 25C is 2.5x10^-9

a) calculate the solubility of CaCo3 in pure water (mol/L and g/100mL)

b) Calculate the solubility of CaCo3 in a solution of 0.2M CaCl2 at 25C.

c) what is the ratio of SO42- to CO32- in a solution at equilibrium with CaSO4 and CaCO3?

For a) I got x= 6.7*10^-5 mol/L and 6.7*10^-4 g/100mL

I did this by

ksp=[Ca2+][CO32-]=[ x][ x]=4.5*10^-9

x^2=4.5x10^-9

and b) I got 2.3*10^-8 mol/L

I did this one by 0.2x=4.5*10^-9

I'm not confident on either of those answers, I just roughly followed the professors powerpoint and even if I did a and b correctly I dont understand why and I am lost on how I should even attempt c)

Thank You