You are going to solve this in such a quick, slick way
that you will blink and not believe it. Fasten your seat belt.
Here are the tools you'll need:
-- Volume of a cylinder = (pi) (radius)² (height)
-- Volume of a sphere = (4/3) (pi) (radius)³
Now ... the question says that your cylinder and your sphere
have the same volume. Fine ! So you can immediately write
an equation saying that their volumes are equal.
(pi) (radius)² (height) = (4/3) (pi) (radius)³
Divide each side by pi: (radius)² (height) = (4/3) (radius)³
Divide each side by (radius)² (height) = (4/3) (radius)
Fill in the given height: (6 cm) = (4/3) (radius)
Multiply each side by 3/4 : (6 cm) x (3/4) = radius
I'm pretty sure that you can take over now and find the radius.