Aright cylinder has a diameter of 30 mm and a height of 30 mm, as shown.
what is the volume of the cylinder?

"As shown" ?  You have a picture to work with but you're not sharing it ?

Fortunately, the question includes all the information that's needed to answer it,
so I don't need to see the picture.

The volume of a cylinder is (pi) x (radius)² x (height) .

Radius = 1/2 of the diameter = 15 mm

Volume = (pi) x (15 mm)² x (30 mm) = 6,750 pi mm³ = 21,205.75 mm³ .

That's a correct answer, but the number is slightly awkward and inconvenient.  Let's
change it to a bigger unit, so the number will be smaller and easier to carry around.

Remember that 10 mm = 1 cm

So (10 mm)³ = 1,000 mm³ = 1 cm³

and 21,205.75 mm³ = 21.20575 cm³