I study mechanical engineering in Germany and somehow I got a problem with calculating with the Thévenin Theorem. I'll write an exam in 5 days so your help is much appreciated!!

I've read the articles on this side but somehow I still get wrong results.

In nearly every task we have to calculate the open-circuit voltage.

So my guessing was:

The open-circuit voltage behaves the same to the resistor it drops of at, as the voltage sorce to the resistors it drops of at. (sorry if I'm translating wrong, but hopefully you'll get my point. I've also posted a picture with the Task and the equations I used.).

Apparently my calculation is wrong. You'll probably see that I don't understand this subject that well but hopefully you'll see what my comprehensive issues are and maybe you can help me.

THANK YOU!