The triangle BCD and ABE are congruent, so AE=DC.
Triangle BMN is an equilateral triangle.
Proof: The main idea is to prove that MN is parallel to AB.
BM is the angular bisector, EM: MA = BE: BA (Angular bisector Theorem 2).
EN:NB = sinECD:sinBCD = SINCDB:sinBCD = BC:BD = BE:BA = EM:MA
So MN is parallel to AB.
BNM angle = 60
equilateral triangle