The net force acting on a car rolling down a ramp is the combination of three forces. one of the forces is the ramp pushing up to support the car. a. . b. which of these two forces the motion of the car? c. which of these two forces acts against the motion of the car?

I've attached a sketch of the problem to better understand it (the image appears rotated by 90 degrees for some reasons)

a) The three forces acting on the car are:
- the weight of the car: , which can be decomposed along the parallel () and the perpendicular () direction
- the normal reaction of the ramp: 
- the frictional force, given by , where  is the coefficient of friction
The direction of the forces can be seen in the graph attached.

b) There is actually one force helping the motion of the car along the ramp: it's its weight , we see in fact that the weight has a component parallel to the plane of the ramp, directed downward, so it helps the motion of the car.

c) there is actually one force that acts against the motion of the car: is the frictional force . We see in fact this force is directed along the plane of the ramp, but upward, so against the motion of the car.