The thrust from a car is 590 N. The air resistance and friction together total 680 N. What happens to the speed of the car?

The net force in the direction the car is traveling (forward) is

(590 - 680) = - 90N = 90 newtons backwards.

The car accelerates backwards at the rate of (90/its mass) m/sec-squared.

Another way to look at it: The car's forward speed decreases at that rate.