Frictional force

This cannot be determined. Although the car is moving at a constant speed, its direction of motion may or may not change.

If the car is also moving in the same direction (uniform motion), then the constancy of motion implies there is no net force. Frictional forces of 800N must act on the car in the opposite direction of the force exerted by the car engine.

However, the car may undergo circular motion. In this case the amount of both tangential frictional forces and radial centripetal forces have to be considered and cannot be found only from the magnitude of the total frictional force. There are cases for which the car experiences frictional forces of 800N or otherwise. The difference in magnitude of radial acceleration will cause the radius of curvature to vary.

When a car is travelling at constant speed, the net force acting on it is zero. Hence, the total resistance force (air resistance and friction) equals to the force produced by the car engine.

In your example, the total resistance force is 800 N provided that the car travels on level ground.