Successive Rotations

Given: triangle ABC and its image A'B'C' with center of rotation D and triangle A''B''C" which is the rotated image of A'B'C' about center E. slider alpha rotates the first transformation, slider beta rotates the second transformation, and slider gamma will be connected to the successive rotations.
Determine the center of rotation for the successive rotations (triangle ABC -->triangle A"B"C") and the degree of rotation. Rotate triangle ABC about the new center of rotation and replace the degree measure with letter gamma to connect the slider to the rotation. Use the slider to see the rotation and verify the degree of rotation.