I assume they mean how many PbX2 has crystallized?
If your answer at 1) is correct, you have a concentration of Pb2+.
You then calculate how many moles of Pb2+ this is.
Your original number of moles of lead you can calculate from the lead nitrate solution you started with.
The difference is the number of moles that reacted
ps. use the numbers and formulas here if you want, then we can have a better look at where you might be going wrong