Question

statements reasons
$mparallel n$ given
$mangle1 = 120^{circ}$
$angle5congangle1$

Explanation:

Step1: Recall parallel - line property

When two parallel lines ($m\parallel n$) are given, corresponding angles are congruent. $\angle5$ and $\angle1$ are corresponding angles.

Answer:

Corresponding angles of parallel lines are congruent.