In order to approach this problem, you need to understand that when sodium chloride (NaCl) is mixed with silver ions (Ag+), a precipitate of silver chloride (AgCl) will form (see balanced equation below). The maximum concentration of Ag+ ions can be determined from the Ksp value (which can be looked up in a table) and the known concentration of NaCl (2.0x10-3 M).
Ag+(aq) + NaCl(aq) ==> Na+(aq) + AgCl(s)
AgCl(s) <==> Ag+(aq) + Cl-(aq) .... Ksp = 1.8x10-10
Ksp = [Ag+][Cl-]
Let x = [Ag+]
[Cl-] = 2.0x10-3 (given)
Ksp = 1.8x10-10 = [x][2.0x10-3]
x = 9x10-8 M = [Ag+]