Get difference between rotations and positions - unity3d

Question

I've got two objects in World Space.
One is a cube has no parent.
The second is a triangle and it has a parent.
I change the cube's position and rotation.
And now I need to put cube at its first position but move triangle in the parent (local) in such position to fit the same position as if the cube won't be placed at the previous position.

Answer 1

Somwhere store the original position and rotation of cube
Vector3 origPosition = cube.transform.position;
Quaternion origRotation = cube.transform.rotation;
Get the offset values between cube and triangle
Vector3 posOffset = triangle.transform.position - cube.transform.position;
Quaternion rotOffset = Quaternion.Inverse(cube.transform.rotation) * triangle.transform.rotation;
(Re)Set cube and Triangle into place
cube.transform.position = origPosition;
cube.transform.rotation = origRotation;
triangle.transform.position = origPosition + posOffset;
triangle.transform.rotation = origRotation * rotOffset;
Example
public class CubeMover : MonoBehaviour
{
public Transform cube;
public Transform triangle;
private Vector3 origPosition;
private Quaternion origRotation;
// Start is called before the first frame update
private void Start()
{
origPosition = cube.transform.position;
origRotation = cube.transform.rotation;
}
[ContextMenu("Test")]
public void ResetCube()
{
Vector3 posOffset = triangle.transform.position - cube.transform.position;
Quaternion rotOffset = Quaternion.Inverse(cube.transform.rotation) * triangle.transform.rotation;
cube.transform.position = origPosition;
cube.transform.rotation = origRotation;
triangle.transform.position = origPosition + posOffset;
triangle.transform.rotation = origRotation * rotOffset;
}
}
(Had no Triangle so I used the Cylinder ... I hope that's fine for you ^^)