If the force is constant then

where v is the average speed.

So to solve your problem, you know that the car is moving at a constant speed of 50.8 mph and the engine is putting out a power of 66 hp. The friction force may be assumed to be constant and is countering the any force the engine is providing, so friction is providing as much stopping power as the engine is putting out. So convert the numbers to the appropriate units and plug them into P = Fv, where F is the friction.

-Dan