Looking at the figure, line ED is a straight line. The angles lying on line ED are
angle EAB = 90 degrees
angle CAB = b degrees
angle DAC = a degrees
Recall, the sum of the angle in a straight line is 180 degrees. This means that
angle EAB + angle CAB + angle DAC = 180
90 + b + a = 180
b + a = 180 - 90 = 90
Recall, two angles are complementary if their sum is 90 degrees. Since angle b + angle a = 90 degrees, then they are complementary angles