A particle moves along a straight line and its position at time $t$ is given by $s(t)= 2t^3 - 21 t^2 + 36 t$ where s is measured in feet and t in seconds.
Find the velocity (in ft/sec) of the particle at the time when the acceleration is 0.
velocity =
The particle changes direction twice from positive to negative at time
$t =$
and from negative to positive at time
$t=$