The Thévénin Equivalent of a circuit consists of a Thévénin voltage source in series with a Thévénin resistor and is valid for any load. In AC circuits a Thévénin equivalent circuit is valid for a single frequency.
The Thévénin voltage is the open circuit voltage at the output. calculation is straightforward node or loop circuit analysis. Using loop analysis:
Voc: [8 * I1] + [(8+2+6) * I2] - [6 * I3] = 1 A
I2 = ½A ==> Voc = ½A * 6
Ω= 3 V = Vth
Calculate the Thévénin resistance from the open circuit voltage and the short circuit current
(set RL = 0 and solve for I3) using Ohm's Law.
Rth = Voc / Isc = 3 V / [(4/9) A] = 27 / 4 Ω = Rth