Answer:
13x + 10
Explanation:
Rafael's hot-water tank holds:

Gallons of hot water.
Rafael used:

Gallons of hot water.
And we want to determine which binomial represent the amount of hot water remaining in the tank.
The amount remaining R in the tank will be the amount taken by Rafael subtracted from the total amount. Hence:

Simplify. Distribute:

Rearrange:

Combine like terms:

In conclusion, 13x + 10 gallons remain in the tank.