What is the purpose of the bolded sentence in the passage the lettermartin luthe?

What is the missing statement in step 3 of the proof? given: z1 is complementary to 22. 22 is complementary to 23. prove: m_1 = m23 m21 = m2 m2 1 + m22 = 90° m22 = m23 m22 + m23 = 180° 2 statements 21 is comp. to 22 2. 22 is comp. to 23 3. ? 4. mz1 - 90°-m22 m22+m23 = 90° 6. m23 = 90°-m22 7 m21 - m23 reasons 1. given 2. given 3. def of comp. zs 4. subtr equality prop. 5. def. of comp. 25 6 subtr. equality prop 17. trans. prop. vo