thevenin resistance

I am not supposed to change the components in the box here. So, to find Thevenin resistance, I added in a 1A current source and figured Voc would be equal to Rt. But Voc keeps coming up as 15, and I checked that no other hidden impedances exist, so why is it coming up like this?

Hello, I am new to solving things using Thevenin's Theorem and I am getting very stuck on a problem. I completed 5 other ones in no time but this one just seems to stump me. I have a program where I can check my answers after drawing out the circuit and I have no idea how to get the numbers...

My approach: We find Thevenin's resistance Using the node analysis approach figure out Thevenin's voltage. And finally the current through the load resistance. Am I doing it correctly? Any suggestion or advice is highly precious to me, thank you