P, Q,R and S are the points of intersection with the coordinate axes of the lines
If the point of intersection of two lines with coordinate axes be concylic, then product of intercepts on the x-axis is equal to product of intercepts on y-axis by these lines.
This is a geometric property.
The intercepts on x- axis are p and q and whose product is pq.
Also, the intercepts on y-axis are p, and q, whose product is also pq.
Hence, the four points are concylic.