Answer:
Since AB is parallel to QR:
Angle A = Angel Q
Angle B = Angle R
(Corresponding angles)
Angle P is common to both triangles, PAB and PQR.
Therefore PAB and PQR are similar figures.
Similar figures have sides in the same ratio:
PA/PQ = PB/PR
Cross multiplying, we get:
PA × PR = PB × PQ