Acceleration = (change in speed) / (time for the change)
Change in speed = (ending speed) - (starting speed)
Change in speed = (60 km/hr) - (50 km/hr) = 10 km/hr
Time for the change = 12 sec
Acceleration = (10 km/hr) / (12 sec)
Acceleration = 0.8333 km/hr-sec
Convert to a unit that we can understand:
Acceleration = (0.8333 km/hr-sec)x(1000 m/km)x(hr/3600sec)
Acceleration = (833.3 / 3600) (m-hr / hr-sec²)
Acceleration = (833.3 / 3600) (m/s²)
Acceleration = 0.231 m/s² or 0.833 km/hr-sec²