Please answer 13 question with detailed explanation........

.....thankiu in advance

Dear student,

Since charge oncapacitor is, the voltage across it is. Since it is in parallel to capapcitor C, C also has 12V across it.

Since the battery provides 18V, the voltage oncapacitor is.
Hence the charge stored on it is. This current must have divided into the two capacitors in series. Hence charge stored on C is. But potential difference across it is. Hence the capacitance C is