1) given
2) definition of linear pair (2 angles with a common ray that for a straight line)
3) definition of linear pair and supplementary
4) definition of supplementary
5)the measures of angles 1, 2, and 3 add to 180
6)substitution involving 4 and 5
7) measure of angle 1 plus angle 2 = measure of angle 4 Subtraction Property of Equality (erase measure angle 3 from both sides
