[tex] {12x}^{4} + {39x}^{3} + {9x}^{2} [/tex]
We csn pull out 3x² out of the expression.
[tex] {3x}^{2} (4 {x}^{2} + 13x + 3)[/tex]
We simply pull out the x-term with lowest degree and 3 out of the expression. If another expression cannot be factored after first factoring then that's the final answer.
[tex] {4x}^{2} + 13x + 3 \\ (4x + 1)(x + 3)[/tex]
[tex]3 {x}^{2} (4x + 1)(x + 3)[/tex]
[tex] \large \boxed { {3x}^{2} (4x + 1)(x + 3)}[/tex]