Find Thevenin equivalent of a 12 V source with 4 ohm and 6 ohm in series, terminals across the 6 ohm. Vth = 12 x 6/(4+6) = 7.2 V (voltage divider). Rth = 4||6 = 2.4 ohm (source shorted). Any load sees 7.2 V with 2.4 ohm series.