need help with this math question

Question

5. Alex is trying to fill his goldfish tank. After filling his tank for one hour it contained 5 liters of water. The tank will gain 1 Liter of water each hour after that. That is the tank will contain 6 liters of water after two hours, 7 liters of water after three hours, and so on. 

1. Find the general term to describe how much water is in the tank after a certain “number of hours” 
2. If the tank has a capacity of 20 liters, how many hours will it take to fill the tank? 

Answer 1

(1,5) (2,6) is a linear relation with slope = 1

Volume = V = H+4 with V measured in liters, H in hours. Initially at time zero, there was 4 liters in the tank

V=H+4

set V=20 and solve for H to find how long to fill the tank

20=H+4

H = 16 hours to fill the tank