Let s be the amount of soda Carla drank and w be the amount of water she drank. We know that s + w = 54 and s = 3w - 6. Substituting the second equation into the first, we get 3w - 6 + w = 54.
Combining like terms, we get 4w - 6 = 54
Adding 6 to both sides, we get 4w = 60
Dividing both sides by 4, we get w = 15
#### 15