Solubility Equilibria calculation

Q3)
Calculate the solubility of AgBr in 1.0 M solution of Na2S2O3
Ksp for AgBr = 5.0*10-13
Kd for [Ag(S2O3)2 ]3- = 1.0*10-13
answer :0.4 mole dm-3

Q4)
The solubility of a slightly soluble salt AB is s mole dm-3
wht would be the molarity of a solution of A+ which just
causes precipitation when mixed with an equal volume of a
solution of B- with molarity 2s?
Answer : 2s

show step plzz =]