If a line meets two parallel lines and is perpendicular to one of them, prove that it is perpendicular to the other.
If two parallel lines are cut by a transversal then the cointerior angles are supplementary.
(If one is x the other is 180-x)
If one is 90 then thte other is 180-90=90.
So if the transversal is perpendicular to one of the lines it must be perpendicular to the other as well.