Determine the time interval on which the point is moving to the right and to the left.

Point P is moving along the x axis so that its position x at time t is given by x=3t^3-4t+1

So I determined the velocity and accleration equations:

v(t) = 3t^2-4

a(t) = 6t

But I have no clue how to determine what time intervals which the point is moving to the right or to the left?