So the actual question is this-
To two plates of parallel plate capacitors, charges $Q_1$ and $Q_2$ are given. The capacity of the capacitor is C. Then the switch is closed. [assume both $Q_1$, $Q_2$ to be +ve]
There were options to this problem but that's not the part of my question.
Once the switch is closed, the potential of the second plate will become zero but the trouble is how can I equate the potential to zero when there is no way of calculating the absolute potential on it. I want to calculate the final charges on both sides of both plates.