The graph shows the amount of water that remains in a barrel after it begins to leak. The variable x represents the number of days that have passed since the barrel was filled, and y represents the number of gallons of water that remain in the barrel. A graph titled Water Supply with number of days on the x-axis and gallons of water left on the y-axis. A line goes through points (6, 33) and (15, 15).

What is the slope of the line?
–2
Negative one-half
StartFraction 7 Over 16 EndFraction
StartFraction 39 Over 30 EndFraction