It is very easy. the subsets of A will be all the sets which you can make with the combination of elements or numbers given in it.there can be many subset.Null set is always a subset of any set.
By subset, we mean that the members of subset are members of the big family and staying separately. they can have any combination .
just like 3 or 5 or 7 or 9 or 12 or 17 can live separately thus forming 6 subsets.
Similarly 3 can team up with 5 or 7 or 9 or 12 or 17 thus forming 5 subsets.
or 5 can team up with o 7 or 9 or 12 or 17 . with 3 it has already teamed up as above and so on.
Similarly 3,5 or 7 can team up together as three members and so on.