The engine of a car delivers a constant power of P to the wheels. The resistive forces are constant at all speeds. The maximum speed that the car can achieve on a level road is v. If the car is going at v/3, the net force acting on the car is

Let the constant resistive force acting on the car be R. At the maximum speed, velocity i.e.K.E. of the car is not increasing further, so the entire power delivered  to the wheels is used up in doing work agains resistive forces. P = Rv ⇒ R = P/v. At time t, let the displacement be s
 

where u = instantaneous velocity. Differentiating both sides w.r.to 't'
 

when u = v/3,       SO     $F_{net}=\frac{p}{v/3}-\frac{p}{v}=\frac{2p}{v}$