Thevenin's theorem in transistor circuit
Perhaps all you need is some visual aid.
Here's your circuit and its equivalent version once I cut the top 22V rail in the middle. (You have to ask yourself why I can cut the circuit between those upper nodes when they both are at the same voltage).

And now I can redraw it so that I have a black box with one port. You can apply Thevenin to that one port and get your final circuit

It's as easy as that.