Engineering Thevenin circuit, voltage problem

[th3plan]

Homework Statement

Ok, i don't understand how they calculate the voltage. how are they getting the Vx? How do i read the circuit to know when to subtract, and add voltages? Also for Voc why do they add them together ?



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[marco75]

Hi,
12A current can flow only into the 4ohm resistor (12V voltage source branch is "open") --> you find Vy.
The same current can't flow into the 9A current generator, so can only flow into the 12ohm resistor -->you find I1 and Va.
By adding (algebraic) Va and Vy you find Vx (KVL) and by adding with 12V voltage source you find Vth.

You'll find Rth shorting voltage source and opening current sources.