Hello, and welcome to FMH!
What is the volume \(V\) of a sphere, in terms of its radius \(r\)?
I don't know. It is not shown from the question.
I haven't done it yet because I don't know how to do it, that's why I'm asking for help here.It's one you should probably commit to memory as you continue your study of calculus. It is:
[MATH]V=\frac{4}{3}\pi r^3[/MATH]
What do you get when you differentiate both sides with respect to time \(t\)?