Answer:
90 (degrees) - a = b
90 (degrees) - (40) = 50 (degrees)
Explanation:
The little box in the corner indicates that it is a right angle, all right angles are equal to 90 degrees.
They gave you the value of (a) and we know that (b) must be equal to 90 degrees when added to (a).
Subtract 90 from 40 and that gives you fifty degrees.