Complementary angles add up to 90°.
You are given one angle that is 63.6°.
Let's make the other angle "x".
63.6 + x = 90
To find x, all you have to do is subtract the other angle (63.6) from 90.
90 - 63.6 = 26.4
x = 26.4
The measure of the complementary angle is 26.4°.
