Hey! I know I've had a lot of questions on here over the past couple days, but I am just ensuring I understand what I am doing, and you guys do such a great job of explaining everything!

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?