Draw a segment connecting the two parallel lines. Draw that segment near the parallel line symbols (the small arrowheads). Now you have a pentagon. The sum of the measures of the interior angles of a pentagon is
(n - 2)180 = (5 - 2)180 = 3(180) = 540
The angles the new segment makes with the parallel lines are same-side interior angles of parallel lines cut by a transversal, so they are supplementary. Their measures add up to 180 deg.
