If the plates of a parallel plate capacitor are given charges 4Q and -2Q . The capacitor is then connected across an uncharged capacitor of the same capacitance as first one (=C) . Now we have to find the final potential difference between the plates of first capacitor .
I tried it and done the charge distribution but not able to proceed .
Your diagram shows that you have almost solved the problem.
You have obviously correctly decided on how the charges ($4Q$ and $-2Q$) are distributed on the original capacitor.
If you double the area of the plates for the first capacitor what happens to the charge distribution on the plates and what happens to the capacitance of the larger area arrangement relative to the original capacitance?
Doubling the area is equivalent to adding the extra capacitor of the same capacitance.
