equilibrium equation is
Ag2CO3(s) <---> 2 Ag+(aq) + CO32-(aq)
From the reaction equation above, the formula for Ksp:
Ksp = [Ag+]^2 [CO32-] = 8.1 x 10^-12
You know [CO32-], so you can solve for [Ag+] as:
(8.1 x 10^-12) = [Ag+]^2 (0.025)
[Ag+]^2 = 3.24 x 10^-10
[Ag+] = 1.8 x 10^-5 M