For a), not sure why you'd have to multiply by 5. There's only one way of getting the four aces, but in poker you've got the 'kicker' (the 'other card') which forms the rest of the hand. Since the other four cards are aces, and it doesn't matter what the kicker is, there are 48 different kickers. The probability is going to be that over the total number of ways, which is going to be 52x51x50x49x48 (picking 5 cards) divided by 5! since it doesn't matter what order the cards appear in (effectively removing the ordering), meaning the answer will be
For b), you can first pick the suit, (of which there are 4), then pick any 5 cards from that suit. Similar to above, to pick 5 from 13 we have 13x12x11x10x9 over 5! which eliminates order. We then have all that over the total number of ways, ie the answer will be
Here, we multiply by four for the four suits
Hopefully this makes sense, and hope this helps