How do you find the equilibrium conc. of Ag^+ in parts per billion that would exist in equilibrium with Ag?

A concentration of 10-100 parts per billion (by m) of Ag is an effective disinfectant in swimming pools. However, if the concentration exceeds this range, the can cause adverse health effects. One way to maintain an appropriate concentration of is to add a slightly soluble salt to the pool. Calculate the equilibrium concentration of Ag in parts per billion that would exist in equilibrium with AgCl. How do you do this?