When proving that all three angles of Triangle ABC add up to 180 degrees. The flow chart is missing a reason underneath the first listed Substitution on the right. The numbered blank space answer would be another Substitution. If your question asks for the final step in the attached diagram then, the answer would be Angle Addiction Postulate.
Read more on Brainly.com - https://brainly.com/question/1615688#readmore (that answer was wrong with definition of supplementary angles)
