A diagram helps.
∆BNL and ∆BML are congruent right triangles (given one angle and congruent hypotenuse BL). So, ∆MNL is isosceles, with angle L being 180° - 40° = 140°
Then angles M and N of that triangle are each 20°.
The angles M, N, L of ∆MNL are 20°, 20°, 140°.
