The linear density is said to give the way the mass is changing it's position and it's a derivatives of mass m here which is differenciated with respect to x position
Given
mass m = 3x^2.
Density ρ= dm/dx =6x
Then if we substitute x=1 into above expresion we have
ρ(1) =6 Kg/m
ρ= 6Kg/m
Hence this is how fast is the mass changing at that position.
