Determining Parallel Resistances in Solution of Circuit by Thevenin's Theorem
Using thevenin's theorem you need to approach your circuit from the open circuit side(Across terminals A and B) rather that from the voltage source as we would normally do. So by looking at the circuit from the right to left we see that the 8k and 4k resistors are in parallel and then the 12k is in series with the parallel combination(8K + 4K). Start at point A and work your way through the circuit