Top triangle: if the know angle measures are 90 and 55 and there are a total of 180, 90 + 55 + a = 180. This means that angle a = 35°
Bottom triangle: if a is 35, the angle connected to it is 35 because they are vertical angles. The angles of this triangle are 35 + 120 + b = 180. Angle b = 25°
