Average acceleration problem having trouble

[robbie8292]

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?

Homework Equations

I have Vi=11m/s, Vf=78m/s and t=1.3 m/s

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.

 

[physics604]

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?

a=(v_i+v_f)/t

a=(11-78)/1.3

a= -51.54 m/s²

Your v_f should have been -78 m/s since it was moving in the opposite direction.