Angles 1 & 2 have to equal 180. So, if ∠1 is equal to 140, ∠2 is equal to 40 because 140 + 40 = 180.
So far... we know that:
∠2 = 40
∠9 = 80
Angles 2, 9, and 11 make a triangle, a triangles "magic number" is also 180. To get 180, you must add up all of the angles. Well... we don't know ∠11 so to find it, you subtract 40 & 80 from 180. 180 - 80 - 40 = 60.
