The total capacity of the water tank is gallons, and the rate of change is gallons per hour.

The total capacity of the water tank is 400 gallons, and the rate of change is 20 gallons per hour. I am not sure of the answer.
Total capacity is 400 because when x=0 then y =400
Rate of change = y2-y1/x2-x1
= 300-400/5-0 = -20 gallons per hour.
Hope this helps.