Speed at the beginning = zero
Speed after 12 seconds = 45 m/s
Average speed during the 12 seconds = (1/2) · (0 + 45) = 22.5 m/s
Distance covered in 12 seconds at an average speed of 22.5 m/s :
Distance = (speed) x (time)
Distance = (22.5 m/s) x (12 sec)
Distance = 270 meters