How can I calculate the average acceleration?

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)

[tex]v_{f} = final velocity[/tex][tex]v_{o} = initial velocity[/tex][tex]t_{f} = final time[/tex][tex]t_{o} = initial time[/tex][tex]a = average acceleration[/tex][tex]a = \frac{(v_{f}-v_{o})}{(t_{f}-t_{o})} [/tex]