0<t≤1s velocity increases from 0 to 6 m/s
1s≤t≤2s : velocity decreases from 6 m/s to 0 m/s
But car continues to move forward and then it reverses its direction at t=2s and after 2 s accelerates backward

Distance undefined
∣21×6×2∣+∣21×(−6)×1∣
=6+3
=9m
Displacement = Total area under v−t graph
=6−3=3m
∴ Average velocity =TimeDisplacement=33=1m/s
∴ Average speed =TimeTotaldistance=39=3m/s