An object traveling 50 m/s comes to a stop in 4 seconds. What is its acceleration

Acceleration = (change in speed) / (time for the change)

Change in speed = (speed at the end) - (speed at the beginning).

Change in speed = (zero) - (50 m/s) = - 50 m/s

Time for the change = 4 sec

Acceleration = (-50 m/s) / (4 sec)

Acceleration = - 12.5 m/s²