How to find \$R_{eq}\$ in this situation?
This is a typical exercise to solve by inspection, but once you find the &quot;disturbing&quot; element.
In this case the disturbing component is the 4k resistor, so please, think about temporarily detaching it from the circuit.
After that, you can see that the two 12k resistors are in parallel and in series to the 6k resistor for a total of 12k.
Now, attach again the 4k resistor (which results in parallel to what you previously found), and finally obtain 4//12 = 3k.