By definition, acceleration is the change in velocity per unit time. In the given problem, the initial velocity is 7 m/s. Since the statement says that it comes to a stop, the final velocity becomes zero. The time elapsed is 3 seconds. Using the equation for acceleration, we substitute the values to solve for a:
a = (v,final - v,initial)/t
a = (0 - 7)/3
a = -2.33 m/s²
