My explanations below is just confined to only brackets, as you have understood the other one.
Here what we required the combination of 3 Aces and 7 diamonds with any other 3 cards, since in Bridge hand a person will hold 13 cards.
Choice-1: The person receives - one diamond Ace + 6 any other diamonds
from the remaining 12 + 2 Aces from the remaining 3 Aces + any 3 cards
from the remaining 36 cards
NOTE: Here one diamond Ace is the intersection of 4 Aces set and 13 diamond card set; hence the selection one diamond Ace with any 6 other diamonds and other 2 Aces, clearly meets the specification of 3 Aces as well 7 diamonds.
=> the combination = C(1,1) x C(12, 6) x C(3, 2) x C(36,3)
Choice-2: The person receives - Excluding Ace diamond the person receives any 7 from the remaining 12 diamond cards + all 3 Aces from the remaining 3 Aces, leaving diamond Ace + Any 3 cards from the remaining 36 cards (4Aces and 12 diamonds are removed)
=> the combination = C(12, 7) x C(3, 3) x C(36, 3)
Wish you are satisfied with the explanation; have a nice time