computing Impedance related with Voltage Vx
I taught this at first year level for a few years, and here's the method I found students had the most success with:


Identify a part of the circuit where two passive circuit elements can be combined.
Combine those elements, calculating the new value.
Redraw the circuit. If you're learning how to do this, don't leave that step out! It's valuable for many reasons, not least that it helps the marker follow your reasoning.
Repeat from 1. until you have simplified the circuit as much as possible.
Perform a source transformation, where you convert a voltage source in series with a resistor into a current source in parallel with a resistor (or vice versa).
Redraw the circuit with the new values.
Repeat from 1. again.


Eventually you'll get down to one voltage source and a resistor in series with it - the Thevenin equivalent circuit. If you end up with a current source in parallel with a resistor you've got the Norton equivalent - perform a source transform to swap between them.

It also helps to draw a circuit diagram that is equivalent to the one you're given, but arranged to make the problem easier. I've had a stab at that in this image: