please explain how to find the concentration?

I have a lab coming up and this is one of the practice questions that we need to do. I have no idea how to do this. I think that we need to plug in all known info into the nerst equation but i don`t know how to arrange Q or which equations to use to find Eocell. If you have any advice I would REALLY appreciate it!

The solutions at the two Pb electrodes of a concentration cell were prepared as follows:

Cell A: A mixture of 1.00 mL of 0.0500 M Pb(NO3)2 with 4.00 mL of 0.0500 M KX (the soluble potassium salt of an unspecified monovalent ion X-).

Some PbX2(s) precipitates.

Cell B: 5.00 mL of 0.0500 M Pb(NO3)2.

The cell potential was measured to be 0.05700 V at 25 °C.

By use of the Nernst equation, determine the concentration (M) of Pb2+ in the solution of Cell A.