We have that

Volume_{sphere} = (4/3)pi (radius)^3

9000 = (4/3)pi (radius)^3 divide both sides by (4/3)pi

9000/ [ (4/3)pi ] = radius^3 takethe cube root of both sides

∛ ( 9000/ [(4/3)pi] ) = radius ≈ 12.9 units

And the diameter is twice this ≈ 25.8 units