To find the size of the missing angle, first form an equation that is equal to 360˚. 𝒂 + 135 = 360. Then make 𝒂 the subject of the equation with 𝒂 = 360 − 135.

Learn about angles in triangles and quadrilaterals with this BBC Bitesize Maths article. For students between the ages of 11 and 14.