The first thing we need to do in this question is to set up the balanced reaction with the physical state of each compound:
Na2CO3 (aq) + Pb(NO3)2 (aq) -> 2 NaNO3 (aq) + PbCO3 (s)
Now that we have the properly balanced equation with the physical states, we can calculate the net ionic equation, which by definition is the equation that will influence directly the reaction, either by changing the oxidation number of the elements or by changing the physical state
In this case, we don't have a change in oxidation number of any element, but we have a change in physical state, with the precipitated salt PbCO3, therefore the net ionic equation will be:
CO3^2- (aq) + Pb^2+ (aq) ---> PbCO3 (s)