Answer
The equilibrium would favor the reactants, producing more $PbCl_2(s)$.
Work Step by Step
This is the equilibrium reaction:
$PbCl_2(s) \lt -- \gt Pb^{2+} (aq) + 2Cl^-(aq)$
When we add $Pb(NO_3)_2$, the common ion effect (because it has $Pb^{2+}$ ions) will act, and the reaction will be more reactant-favored.
That follows the Le Chatelier's principle. Since we are raising the concentration of one of the products, the equilibrium will try to reduce this concentration, to reduce the change.
