There are two paralell lines, one is part of Angle B, and the other of Angle Q. Then there is a diagonal line, coming at the same angle from B all the way to Q, because the other two lines are paralell. It basically means if you were to slide the flat line which is connected to Q up the diagonal line, it would fit perfectly with it's parallel. So the two angles; Angle B and Angle Q; are the exact same.
Hope this helped! Good luck with future math problems :)
