Speed of thief 12 km/hr
speed of train 60km/hr
Since tge thief is running inside a moving train,Resultant speed of thief=60+12=72km/hr.
Speed of police car=80km/hr
Speed of thief wrt police car=speed of thief speed of police car
For considering speed which is a scalat quantity,negative sign not needed.
Speed =8 km/hr
Like if satisfied