If $p$th, $q$th, $r$th and $s$th terms of an A.P are in G.P. Then show $p-q$, $q-r$, $r-2$ are also in G.P.

Question

If $p$th, $r$th and $s$th terms of an arithmetic progression are in a geometric progression, then show that $(p-q), (q-r)$ and $(r-2)$ are also in a geometric progression.

I want to understand the middle steps of the question please help