Obviously, measure of angles 1 and 2 are congruent.
The possible reasons are (one, any two, or all of the listed below):
1) The given statement in the problem. Write "Given" as the reason.
2) the previous statement in the Proof column.
3) Definition/Postulate/Theorems/Property, depending on the given or previous statements in the Proof column. This could be Vertical Angles Theorem, Reflexive Property, Definition of base angles of Isosceles Trapezoid, etc.
TIP: Draw the triangle based on given, that is, if an illustration is not provided. Mark the congruent sides and angles, if you are proving congruence.
List down all the theorems and their corollaries (if any), and the definition/axioms/postulates. Draw a figure. Write this is separate paper or hard paper, laminate if you want, practice solving problems that uses as Proof.
(There is no excuse for not knowing how to solve a problem. It takes practice, and reviewing solved problems, and of course, lecture given by your teacher.)