Answer:

(b) There are 7 gallons left in the tank after 4 minuted
Explanation:
Given

time since water began draining
gallons in the tank
Solving (a): Notation for gallons remaining at 4 minutes
This means that

becomes

Solving (b): Interpret f(4) = 7
We have:

time since water began draining
gallons in the tank
This means that:


It can be interpreted as:
There are 7 gallons left in the tank after 4 minuted