A long cylinder of radius a = 6m and length L is made of insulating material and is coaxial with the z - axis ( which points out of the page ) . Surrounding it , and also coaxial with the z - axis, is a cylindrical shell made of metal; it is of the same length L , has inner radius b = 12m and outer radius c = 15m . ( Both cylinders are so long compared wit their radii that they may be considered of infinite length. ) Line - charge densities of λ insul = + 4n C m and λ metal = - 12n C m are placed on the inner cylinder and outer shell respectively. The charge is uniformly distributed along the volume of the insulating cylinder. What is the line - charge density on the outer surface of the conducting shell?