It can help to draw a diagram.
∆LBM ≅ ∆LBN by the hypotenuse-angle (HA) theorem of congruence of right trianges. Then LM ≅ LN and BM ≅ BN by CPCTC. Quadrliateral BMLN is a kite, so diagonal MN ⊥ BL.
Of course, angles BLN and BLM are the complements of the halves of bisected angle B, so are both 70°. And angles LMN and LNM are the complements of those, so are both 20°.
∆MNL is an isosceles triangle with base angles of 20° and an apex angle of 140°.
