What 7(b) means is that a force of 50N is working against the speed of the car since friction helps prevent objects from slipping and it also reduces their speed.
So, all you need to do is subtract 50N from your resultant force on Part A.
You found your resuitant force by doing F=ma.
F = 1000 x 4
F = 4000N
Now, just subtract 50N from it and this will be the resultant force of your car on a rough road with a friction of 50N.
F = 4000N - 50N
F = 3950N
