The first thing you need to do is show that all four points are collinear which is a simple matter of pointing out that the midpoint of a segment has to be on the segment by definition of the midpoint of a segment.
Then, also using the definition of a midpoint, you show that PQ = QR and that QR = RS. Then by transitive equality PQ = RS.
John
My calculator said it, I believe it, that settles it
