Given: Angle1 is complementary to Angle2. Angle2 is complementary to Angle3.
Prove: mAngle1 = mAngle3
3 separate angles are shown. The angles are labeled 1, 2, 3 from left to right.

A 2-column table with 7 rows. Column 1 is labeled statements with the entries angle 1 is complementary to angle 2, angle 2 is complementary to angle 3, question mark, measure of angle 1 = 90 degrees minus measure of angle 2, measure of angle 2 + measure of angle 3 = 90 degrees, measure of angle 3 = 90 degrees minus measure of angle 2, measure of angle 1 = measure of angle 3. Column 2 is labeled reasons with the entries given, given, definition of complementary angles, subtraction equality property, definition of complementary angles, subtraction equality property, transitive property.

What is the missing statement in step 3 of the proof?

mAngle1 = mAngle2
mAngle1 + mAngle2 = 90°
mAngle2 = mAngle3
mAngle2 + mAngle3 = 180°