Answer: Yes; To 17 gal or gallons(depends on the question)
Step-by-step explanation: Whenever you're dealing with math where two things are of the same unit, you are allowed to add them together. In this case, both numbers are associated with gal/gallons, and therefore their Coefficients(numbers in front of the unit) can be added together. You just add the 8 and the 9 together and get 17 gal or gallons.