Answer: The missing angle is 69°
Step-by-step explanation: When all the angles have their vertex on a straight line, they must add up to 180° They are "supplementary" angles.
The "box" in the corner of an angle is a symbol for "right angle" so that is a way to have a "given" 90°
Knowing that, you can add the "given" 90° + 21° to get 111°
Subtract 111° from 180° to find Angle a:
180 - 111 = 69
