Notice that the rate at which the water flows is constant. Let;s call such R.
Then, the equation that represents the number of buckets as a function of time must include this info:
Number of buckets = Rate * time (in minutes)
3 = R * 14
Then R = 3 buckets over 14 minutes.
Number of buckets = 3/14 * time
This is the linear equation that represents the number of buckets filled as a function of the time elapsed (in minutes).