Full houses are ranked amongst each other by first looking for the hand
with the highest ranked Three of a Kind portion. If two hands have equally
ranked Three of a Kind portions, a higher ranked Pair is used to decide
the winner.
Since player 1's Three of a Kind portion has a higher rank (i.e. Aces)
than player 2's Three of a Kind portion (i.e. Nines), player 1 has the
higher ranked hand.
Both player 1 and player 2 have equally ranked Three of a Kind portions
so the Pair portions of their hands must be considered. Since player 2's Pair
(i.e. Nines) is higher ranked than player 1's Pair (i.e. Sevens),
player 2 has the better hand.
Both player 1 and player 2 have the exact same hand, Full House, Aces Full of
Sevens, and as a result both players tie for having the best hand.