For Circuit $4,$ consider that the switch was connected between t $=$ and t $=0.$ At t $=0,$ the switchopens. Answer the following: $($a$)$ Determine the equivalent capacitance, Ceq, of the capacitors in the circuit. $($b$)$ Calculate the current ix$($t$)$ for t $0,$ determine the time constant of the circuit, and computethe energy stored in the equivalent capacitor Ceq at t $=0,$ denoted as wc$(0).$