How to find the rotation matrix between two objects?

Question

Mathematics

Hello. I have an Object, let's say "Fork". I know its initial coordinates. Position and rotation matrix.
Then I rotated it and I also know the final results and the rotation matrix.
How to find now how many degrees the fork was rotated in euler angle.
I think it's like in the picture.
Matrix X came out. I inserted it into the converter. On that website.
https://www.andre-gaschler.com/rotationconverter/
Good angles come out, real ones. But how does the converter calculate the second rotation matrix? That is, on the left ..
Or am I doing everything wrong?
Help me please.

Fork Matrix A

[  0.186812,  -0.908758,   0.373175;
  -0.954698, -0.0783737,   0.287069;
  -0.231629,  -0.409897,  -0.882231]

Fork Matrix B

[ 0.291274, -0.888062,  0.355676;
 -0.933193, -0.181957,  0.309906;
 -0.210498, -0.422182, -0.881733]

Answer 1

Use the dot product of vectors to express the angle (forks are vectors).