it's difficult because you are looking too hard. think simply. why does it tell you that angle is 40°?
Because if you follow the arrow back to where it starts you will see that exact same angle.
so now you know 1 (not m° or n°) angle inside the triangle is 40° and a complete triangle is 180°.
you also know a straight line has 180° on each side. and the line going from m° to n° splits that in half. meaning there is 90° to the right of it and 90° to the left of it. which means n° must equal 90°.
now you know you have 1 angle inside the triangle with 40° and n°=90° now you just need to find how much m° is.
