$v_{av} = \frac{s}{t}$ - this is an average, velocity may not be constant!
$v = u + at$
$s = ut + \frac{1}{2}at^2$
$v^{2}=u^{2} + 2as$
$s$ = displacement
$u$ = initial velocity
$v$ = final velocity (not $v_{av}$)
$a$ = acceleration
$t$ = time