PHYSICS problem! please help asap?

a bicyclist is coasting down an 8degree hill @ a constant velocity of +8.9m/s. the m of the bike and rider is 85kg. a force due to air resistance is directed opposite to the motion & has a magnitude f(air) that is proportional to the cyclist's speed v, so that f(air)=cv, where c is a constant. using the fact that the cyclist is coasting @ a constant v, determine the numerical value(including units) for the constant c. I do not even know how to start! please help