In a circuit with resistors and capacitors, the values of the resistors are given as r1 = 2 ohms, r2 = 6 ohms, and r3 = 9 ohms, and the values of the capacitors are given as c1 = 3 farads and c2 = 6 farads. What is the energy stored in capacitor c2 (in joules) when the circuit is in a steady state?