At a certain time a particle had a speed of 11 m/s in the positive x direction, and 1.3 s later its speed was 78 m/s in the opposite direction. What was the average acceleration of the particle during this 1.3 s interval? 2. Relevant equations I have Vi=11m/s, Vf=78m/s and t=1.3 m/s 3. The attempt at a solution My attempt at the solution has been the formula of (78-11)/1.3. This has gotten me the answer of 51.54 and I at am a real loss. I have read my book over and over and I juts need some guidance.