Hello from MrBillDoesMath!
Answer: Relies on "vertical angle theorem" and sum of angles in a triangles is 180.
Discussion:
As angle(C) = angle(D) (given) each triangle contains 1 identical angle. But angle(A), shared by both triangles, is a vertical angle so both triangles share this second identical angle. Now, a triangle has 180 degree, and both triangles already have 2 identical angles, the remaining third angles in each triangle ( i.e. angles, E, and B) are identical.
For a clearer visualization of this, place a single tick spanning angles C and B, and two ticks spanning angle A in each triangle.
Thank you,
MrB
