Answer :
You know two points on the line:
(0 minutes, 0 gallons) and (8 minutes, 4 gallons) .
The slope of the line is
(4 gallons - 0 gallons) / (8 minutes - 0 minutes) =
(4 gallons) / (8 minutes) =
(4 / 8) (gallons / minutes) =
1/2 gallons/minute .