How to calculate the magnitude of the average net force?

A 1370-kg car coasts on a horizontal road with a speed of 16.7 m/s. After crossing an unpaved, sandy stretch of road 28.1 m long, its speed decreases to 14.0 m/s. Calculate the magnitude of the average net force on the car in the sandy section?How to calculate the magnitude of the average net force?
You first need the deceleration time. Use the following kinematic equations:



v=v0+at, or a = (v-v0)/t



x = x0 + v0t + (1/2)at^2



Substituting the first into the second:



x = x0 + v0t + (1/2) (v - v0)t

= x0 + (1/2)(v + v0)t



t = 2(x - x0)/(v + v0)



where:

x = final position (28.1m)

x0 = initial position (0m)

v = final velocity (14.0m/s)

v0 = initial velocity (16.7m/s)



Once you have that, go back to the first equation to get a:

a = (v - v0)/t



Then, you just need:



F = ma



where:



m = mass of car (1370kg)