<p>well all the angles in a triangle add up to 180 right? so if your given two angles and your trying to find an unknown one you simply subtract the sum of those angles from 180. So the missing angle of the bottom left triangle is 180-(A+b) and when you distribute the negative thats 180-A-B and the missing angle of the middle triangle is also 180-A-B. notice that a straight line also has a angle measure of 180 and the left side of thee bigger triangle is basically a straight line split into 3 parts, and you already have 2 of them. You know 2 of them now and you need to find the third so you know all three angles in the top triangle(the triangle that contains C). So to find it you do 180-[(180-a-b)+(180-a-b)] or simply 180-2(180-a-b). Simplify and you get 180-(360-2a-2b) and simplify further to get -180+2a+2b. Well again 180 minus the sum of this angle and b has to equal the third angle C. So we do 180-(b+(-180+2a+2b)) and that simplifies to 360-2a-3b which is E. This is a confusing problem so I’m sorry if my explanation wasn’t clear</p>