Question 190001
First, start simplifying from inside the bracketed parantheses by distributing the 3:

3x^2 - x [ 4 - ((3 * x) - (3*4))]

Next, distribute the negative sign and combine the 4 and 12:

3x^2 - x [ 4 - 3x + 12 ]
3x^2 - x [ -3x + 16 ]

Next, distribute the -x (all in one step this time for quickness sake):

3x^2 + 3x^2 -16x

6x^2 - 16x

You can further simplify by pulling out a 2x:

2x ( 3x - 8)

:)