rotation – Rotate & Translate a MatrixTRS

Hello I have a question about translation & rotation of a MatrixTRS…

So basicly I have a given MatrixTRS A in space, define by a positionA, rotationA and scaleA.
(DISCLAIMER: I am using Unity, but I do not use Transform, therefore I have a Matrix4x4, created like that:
Matrix4x4 A = Matrix4x4.TRS(position, rotation, scale);

Ok, Now I have a Point in space, with a given rotation, named PositionB, and RotationB.
I would like to translate & Rotate my TRS matrix to that positionB, and rotationB.

I know I have to do something like b.transform.TransformPoint() and Quaternion.Inverse(b.transform.rotation)*a.transform.rotation, but i didn’t manage to make it right yet.