Let a = initial time, b = final time
average velocity is V = (1/(b-a))*Integral(v(t)dt)
where the limits of the integral are a to b.
When you're given s(t), your job is made easier because the above equation turns into
V = (1/(b-a))*Integral((ds/dt)dt) = (1/(b-a))(s(b)-s(a))
I hope everything is clear.. I've been finding LaTeX a little cumbersome lately so I sometimes use it, sometimes not.
Someone please correct me if this is wrong as I have not worked with these for a while.