Of course this is enough, but you must know:
i) If H,K are two sbgps. of a group G, then HK is again a sbgp. iff HK = KH
ii) If N is a NORMAL sbgp. of a group G, then NK is always a sbgp. of G for any sbgp. K of G
iii) For any two sbgps. of an ABELIAN group G, HK is again a sbgp. of G.
iv) For H,K sbgps. of G, |HK| = |H||K|/|H /\ K|
Take some minutes to fully understand the above and then try to put things together.
Note also that (i ==> (ii) ==> (iii)