Question

if the charge of 10 microcoulomb and -2 microcoulomb are given to two plates of a capacitor which are connected across a battery of 12 V, find the capacitance of the capacitor​

Answer 1

Once the capacitor is charged and disconnected from the voltage source, the charge is not going anywhere, it will be conserved (ignore the leaking current due to finite resistance of a dielectric since it’s not known from the problem specs). The second cap is a red herring (assuming it had zero charge to begin with). Energy will be lost (there will be sparks when you connect the second cap) but not the charge. It will only distribute between capacitors in proportion to their capacitances but the total charge won’t change.

Q= 50 V * 80 uF = 400 uC