How do I set when the object isn't moving

I started studying instantaneous velocity derivatives using only now. It may seem stupid but really I'm not sure whether that's right:
I have an equation:
$$x (t) = 1.5t - 9,75t³$$ To set the time when this object stops would velocity = zero , right? If velocity is v (t) = -29,25t² (v = dx / dt = - 9,75*3t²), then the object will be stopped with t = 0s? It's the second equation in which it occurs so I came resort here.

-

First of all, derivative of $1.5t-9.75t^3$ is $1.5-29.25t^2$, you missed first part.