The alternate exterior angles are those angles that are found by the symmetry that is created between parallel lines.
In the image the angles <2 and <3 are equal because they are opposed by the vertex.
and due to the symmetry of the parallel lines the angle 3 <3 is corresponding to the angle 7 <7 (they measure the same for being on the same side in the lines that are parallel)
This means that consequently, angles <2 and <7 are equal and are alternate exterior angles.
