how to find the diameter of a planet in cubic miles from the volume?

Cubic miles = (4/3)pi * radius^3        divide both sides by (4/3)pi

Cubic Miles / [ (4/3)pi ] = radius^3          take the cube root of both sides

( Cubic Miles / [ (4/3)pi) ] ) ^(1/3)  = radius

Double the radius and this will be the diameter