The Mass of Part of a Wire Is $x ( 1 + \sqrt { x } )$

The mass of part of a wire is $x ( 1 + \sqrt { x } )$ kilograms, where $x$ is measured in meters from one end of the wire. Find the linear density of the wire when $x = 36 \mathrm {~m}$ .

A) $6 \mathrm {~kg} / \mathrm { m }$
B) $4 \mathrm {~kg} / \mathrm { m }$
C) $9 \mathrm {~kg} / \mathrm { m }$
D) $1.5 \mathrm {~kg} / \mathrm { m }$
E) None of these