A box of mass 7. 70 kg is initially at rest on a rough surface. At =0, a constant horizontal force of 25. 0 N is applied to the box over a period of 4. 20 s, causing it to reach a final speed of 9. 50 m/s. If another box of mass 1. 30 kg is placed on top of the first box (assume that static friction keeps the top box attached to the bottom box), and then the same horizonal force is again applied to bottom box, what is the acceleration of two boxes as they move together?
a = m/s