If the moving object is undergoing constant acceleration, then
[tex]v_{\rm avg}=\dfrac{v_i+v_f}2[/tex]
(take the average of the initial and final velocities, if they're given)
If you don't know the acceleration or initial/final velocities, then you have to resort to the usual definition,
[tex]v_{\rm avg}=\dfrac{\Delta x}{\Delta t}[/tex]
where [tex]\Delta x[/tex] is the change in position and [tex]\Delta t[/tex] is the duration of the interval.
