Okay, I have a derivative question that I am not entirely sure how to solve, so I was hoping you guys could help me out!

**Question**

The position function of an object moving along a straight line is given by the function \(\displaystyle s(t) = t^3 - 15t^2 + 48t - 10 \), where s is in metres and t s in seconds and \(\displaystyle 0<t<15 \).

i) When is the velocity the object greater than 21 m/s?

ii) When is the speed of the object less than 21 m/s?

**Question**

I really have no idea how to solve for this! With the equation given I know the distance in metres after t seconds, but not the velocity, and the question is asking for velocity.

How would I go about solving this question?