plug the given numbers into the formula

divide both sides by the resulting coefficient of r^3

take cube root of both sides

(on calculator, raising to the 1/3 power takes cube root, use 0.333 for 1/3)

That means rearrange the equation to solve for r. What do you need to know?

9 = (4/3) pi r^3

Multiply both sides by 3/4. That cancels out the 4/3 on the right.

Divide both sides by pi. The cancels out the pi on the right, leaving only r^3. And can you figure out what to do next?