Answer:
Part a) 10 gallons
Part b) 4.59 hours
Explanation:
The picture of the question in the attached figure
Part a) How much water left the tank during the 6 hours shown?
To find out how much water left the tank during the 6 hours, determine the area of the graph in that interval
so
Calculate the area of the rectangle plus the area of the triangle


Part b) How many hours did it take for 9 gallons to leave the tank?
we know that
In the interval [0,4]
8 gallons of water leaves the tank in 4 hours (remember that the area in that interval is equal to 8 gallons)
so
Find out in the interval (4,6] how many hours did it take for 1 gallon to leave the tank
First determine the equation of the line in the interval (4,6)
we have the points (4,2) and (6,0)
The slope is equal to

The equation of the line is

Determine the area of graph in the interval (4,x) (Is the area of trapezoid)
so



For A=1 gal


solve the quadratic equation by graphing
The solution is x=4.59 hours
see the attached figure N 2