The equation 5) I was talking about is:
[H+]+[Ca2+]=[C2O42-]+[H2C2O4]
In both cases (mine and the "right answer") [H2C2O4] can be neglected because it is very small. I meant by solubility the concentration of [C2O42-], when I put it in the equation 5) to calculate [Ca2+], and then multiply [Ca2+] and [C2O42-], I get the value of Ksp same as it is given in the problem. When I put their answer, I get that Ksp=5.77*10-9. So, their answer should be wrong then.
Only one issue: Is it more correct to use the concentration of Ca2+ or the one of C2O42- as the solubility?