A 2.0-mF capacitor is charged to 12 V and then discharged through a 4.0 ˛ 106 W resistor. How long will it take for the voltage across the capacitor to drop to 3.0 V?

Kirchhoff's junction rule: At any junction point, the sum of all the currents entering the junction must equal the sum of all the currents leaving the junction.
Kirchhoff's loop rule: The sum of the changes in potential around any closed path of a circuit must be zero.