On a speed-time graph, a horizontal line shows the change in speed is

On a speed/time graph, the height of the line at any point
shows the speed at that moment.  If the line is horizontal,
then its height isn't changing, meaning that the speed isn't
changing.  It's constant.  The change is zero, until the line
starts rising or falling.

Hey hope you get the answer but i suck at math sorry : (

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.