Verified Answer :

\(v_{f} = final velocity\)

\(v_{o} = initial velocity\)

\(t_{f} = final time\)

\(t_{o} = initial time\)

\(a = average acceleration\)

\(a = \frac{(v_{f}-v_{o})}{(t_{f}-t_{o})}\)

AL2006

Average acceleration =
             (change in speed over some time) / (time for the change)
(could be positive or negative)

Change in speed = (speed at the end) - (speed at the beginning)
(could be positive or negative)

Time for the change = (time at the end) - (time at the beginning)
(always positive)