If s(t) = (1/3)(t^3) - (3/2)(t^2) - 18t

During what time is the object moving towards the origin?

I know I set s(t)v(t) < 0, but from there on I’m a little stuck.

When the velocity is 10m/s, find its displacement.

My answers to this were : -511/6, and 208/3

Is this correct?

How many times does the object cross a marker on the line located at s = 20?

I’m not sure how to do this one.

Any help is appreciated.