I'm pretty sure I need to find Rth using a test source but I can't seem to do it right. (A web form tells me my answer is wrong)
What I tried, I added a current source of 5A to node A. And deactivated the 12V source. The next step for me is to find Vx so I can add it up with R2 * 5A and get Vth. In the end I'll get Rth from, Vth/Ith. But I have failed to get a right answer.