Position of particle moving along x-axis is given as x = (t-3)2, where x is in meter and t is in seconds. Distance covered by the particle in 6 seconds:
At t=0 s ; x=(0-3)2 = 9m
At t=3 s ; x=(3-3)2 = 0m
At t=6 s; x=(6-3)2 = 9m
Distance covered = 9+9 = 18 m