12 gallons.
When Mia turns on the faucet, this is when time = 0, since she just started. X represents the time since the faucet was turned on, and in this case its 0. Substitute 0 into the equation y = 3x + 12:
y = 3(0) + 12
y = 12
Therefore, y (amount of water in gallons) is equal to 12.
Basically its asking how much water was in there to start with.