The Attempt at a Solution
I tried using source transformation.
I simplified the original circuit to the above circuit. I got Rnorton, however, I am a bit unsure on getting Isc. Is it possible to source transform the above circuit further? Or would I have to use KVL/KCL? So I got Rnorton but how would I get I_short_circuit? Is my transformed circuit even correct? Can I really add the two current sources, is that allowed? I am a bit unsure about it.