You simply try all possibilities, for instance 2|4, so the ordered pair (2,4) belongs to PD, but de pairs (4,2) and (2,5) do not.

If I have not overlooked something, you should get these pairs (but try to do this on your own first, to see whether you really understand it).

PD={(2,4),(2,6),(2,12),(2,18),(4,12),(6,12),(6,18) }.