so now this answers my question and You are talking about an assembly where You have placed the part into.
But again, this will bring me to my next question: "Where is the problem now?"
What I did not understand was the fact, that I was able to modify two of the three marked dimenstions of the triangle (You see it in the video), while the third dimenstion was not inclined to offer me the little window, where I could insert the new value for the length.
this was claer from the beginnning.
Look at the colors of those dimensions.
You will find two of them in blue which is indicating "we can be modified by either chaning the dimension value or by clicking the geometry with the mouse and move it so" while the third dimension is purple.
This shows "I am a read only dimension with no chance of chaning anything!"
So before You ask me back, "why?"
This must be a reason of Your part and of the way You have dimensioned this third piece of geometry
And to answer another following question.
The third color which might be there is red, which means this dimension ONLY can be change with modifying the value itself but not moving geometry
Yes red occurs if I set the dimension as fixed, that's what I learned already.
Ok, and You say: The purple color is caused by a reason defined within the file, where I took the part from.
How I can select a center point of a rectangular or the middle point of line and make another 2D object symmetric to that point in one dimension?