Consider the combination circuit below. Each capacitor has the capacitance C $=$

$10\mathrm{.}0$ F$.$ The electric potential from point a to point b is Vab $=12$ V$.$

$($a$)$ What is the equivalent capacitance of the circuit?

$($b$)$ What is the charge on C$4?$

$($c$)$ What is the potential across C$3?$