Final answer:
The coefficient in the expression 3X + 6 represents the rate at which the boat is filling with water at a constant state.
Step-by-step explanation:
The coefficient in the expression 3X + 6 represents the rate at which the boat is filling with water. In this case, the coefficient is 3. This means that for every minute that passes (represented by X), the boat is filling with 3 gallons of water.