What is the missing reason in step 5?

Two angles are complementary if the sum of their angles equals 90o.

If one angle is known, its complementary angle can be found by subtracting the measure of its angle from 90o.

Two angles are supplementary if the sum of their angles equals 180o.

If one angle is known, its supplementary angle can be found by subtracting the measure of its angle from 180o.