Between time 3 second to 20 seconds the time difference = 17 seconds
Distance traveled = 385 - 25 = 360 m
So average velocity = 360/17 = 21.18 m/s
Change in velocity at t =3 seconds and t = 20 seconds is given by 45 - 11 = 34 m/s
So average acceleration = 34/17 = 2 m/s