if a car generates 18 hp when traveling at a steady 88 km/h, what must be the average force exerted on the car due to friction and air resistance

Answer 1

If the proper units are used (Watts, m/s and newtons, for example),
Power = Force * Velocity

First convert to SI units:
88 km/h = 24.44 m/s
18 Hp = 13,428 Watts

Force = P/V = 549 newtons

Answer 2

13,428/24.4= 5.5x10^2N