Physics ,very short question,potential difference,please help

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In this question time constant is 12 mili seconds and C_T_ =9 micro farads and R_T_=1.33 K ohms

* A potential difference of 9V is placed across the capacitors.
a.How long would it take until the voltage across them has been reduced to 6V?
b.How long until the voltage is 3V? Sketch a function of capacitor voltage versus time.


Thanks

a. For discharge we use the formula:

Vcurrent = Vinitial x (e^(-t/RC))

6 = 9(e^[-t/RC])

R = 1.33 x 10^3
C = 9 x 10^-6

t = -(RC)ln(2/3)

t = 4.8 x 10^-3 s

Use the same formula for b.

Capacitor voltage versus time, during discharge, should be inversely proportional.