Average acceleration = (Final velocity - Initial velocity)/time
By putting values in above formula:
Average acceleration = (25 - 0)/8
= 25/8
= 3.125 m/s²
Hence an object moving from 0m/s to 25m/s of velocity in 8.0s has an average acceleration of 3.125 meter per second square. Acceleration is the rate of change of velocity. And average acceleration is equal to change in final velocity and initial velocity divided by time.
Based on the information in the question, a light year is the distance traveled light in one year. Then if the light from the star Centauri takes 4.2 years to reach the earth, then its distance from earth is 4.2 light years.
