When a car is 200 meters from its starting position traveling at 30. m/s., it starts braking and comes to a stop 350 meters from its starting position in 10. s. calculate the acceleration.

A = change in velocity / change in time 

in your case a = 0(final velocity)-30(initial velocity)/10 

so, a = -30/10 = -3 m/s^2

I hope this helps you! 

Is a  vector quantity  that refers to "the rate at which an object changes its position." imagine a person moving rapidly - one step forward and one step back - always returning to the original starting position. while this might result in a frenzy of activity, it would result in a zero velocity. because the person always returns to the original position, the motion would never result in a change in position. since velocity is defined as the rate at which the position changes, this motion results in zero velocity. if a person in motion wishes to maximize their velocity, then that person must make every effort to maximize the amount that they are displaced from their original position. every step must go into moving that person further from where he or she started. for certain, the person should never change directions and begin to return to the starting position.