Let us suppose that lines m and n are parallel lines and a transversal line intersects these parallel lines.
We know that whenever any transversal line intersects two parallel lines then corresponding angles and vertical angles are congruent (equal).
From the figure, we see that angle 2 and angle 6 are corresponding angles.
From the figure, we see that angle 2 and angle 3 are vertical angles.
Therefore, from the above two relations, we see that
