To determine the concentration of the sulfate ion in equilibrium, we can use the ICE table.
Ag2SO3 = 2Ag+ + SO3^2-
I - 0 0
C - 2x^2 x
-------------------------------------------
E - 3.40 × 10-3 x
2x^2 = 3.40×10-3
x = 0.0412 M = [SO3^2-]
Hope this answers the question. Have a nice day