Answer
Friction and air resistance exerted an average force of 508 N.
Work Step by Step
We can convert the power to units of W:
$P = (18~hp)(\frac{746~W}{1~hp}) = 13,400~W$
We can find the average force exerted by the car as it traveled 95 km in 1 hour:
$\frac{F\cdot d}{t} = P$
$F = \frac{P\cdot t}{d} = \frac{(13,400~W)(3600~s)}{95,000 ~m}$
$F = 508~N$
Since the car was traveling at a steady speed, the force of friction and air resistance must be equal (in magnitude) to 508 N.
