The Mass of the Part of a Metal Rod That $x$

Multiple Choice

The mass of the part of a metal rod that lies between its left end and a point $x$ meters to the right is $S = 4 x ^ { 2 }$
Find the linear density when $x$ is $3 \mathrm {~m}$ .

A) 20
B) 24
C) 18
D) 12
E) 4