You don't need it
For an equilibrium expression, remember that what is used in the equation can be either concentrations in solution or partial pressures. Here, it would be concentrations in mol/L (molarity). This is a redox equation which is balanced, but the one product you don't have a concentration for, Ag, is a solid. As such, it won't have a concentration, and doesn't show up in the equilibrium expression.
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