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?

Let v be Davids velocity...

v(t)=6-t

now he is rowing upstream so the velocity of the boat is decreased by the flowing river's rate of 2mph.  

Essentially the boat's velocity is a result of David's speed minus the speed of the river...

b(t)=v(t)-r  b=boat velocity, v=David's velocity, r=river velocity...

b(t)=6-t-2

b(t)=4-t  oh, sorry they want t to be x :)

b(x)=4-x