I'll start with the distance covered while decelerating. Since you know that the initial speed of the car is 15.0 m/s, and that its final speed must by 10.0 m/s, you can use the known acceleration to determine the distance covered by
on one side of the equation and solve by plugging your values
To get the time needed to reach this speed, i.e. 10.0 m/s, you can use the following equation
