David is rowing a boat upstream. the river is flowing at a speed of 2 miles per hour. david starts rowing at a speed of 6 miles per hour, but as he gets tired his speed decreases (at a rate of 1 mile per hour, every hour). which equation represents the speed of the boat for x hours spent rowing?
a. y = x + 4
b. y = 6 − 2x
c. y = 4 − x
d. y = x − 4
e. y = 6 − x