In the figure, we have V = 9.0 V, C2 = 3.0 µF, and
C4 = 4.0 µF, and all the capacitors are initially
uncharged. When the switch is closed, a total
charge of 12 µC passes through point a and a
total charge of 8.0 µC passes through point b.
What is capacitance C3 (µF)?