How to get Quaternion from Transition Matrix

Hello, I'm new on this forum, and i'm new in Unity world, so may be my question has been asked many times, sorry if it's the case. I know the coordinates of 3 points in 2 repositories. I calculated the transition matrix of the référential R1 to R2. So I can calculate the coordinates of any point of R1 in R2 and vice versa. However, I don't know how to obtain the rotation quaternion between R1 and R2. I have seen the functions allowing to pass from a rotation matrix to a quaternion, but here it is a transition matrix. This matrix makes it possible to express the coordinates of a vector with the components of the vectors defining the other référential. I don't find the solution, can you help me please ? any idea ? David.

PS : it's the second time i post this question, but i think there was a problem with my first post, i can't access to my first post.