Remember Your Groups and Squares

Here is the solution to the problem:

I did it by grouping it:

$\quad \quad \quad { a }^{ 3 }\quad -\quad 3{ a }^{ 2 }\quad -\quad 4a\quad +\quad 12\\ \Rightarrow \quad (a^{ 3 }\quad -\quad { 3a }^{ 2 })\quad -\quad (4a\quad -\quad 12)\\ \Rightarrow \quad a^{ 2 }\quad (a\quad -\quad 3)\quad -\quad 4\quad (a\quad -\quad 3)\\ \Rightarrow \quad { (a }^{ 2 }\quad -\quad 4)(a\quad -\quad 3)\\ \Rightarrow \quad (a\quad -\quad 2)(a\quad +\quad 2)(a\quad -\quad 3)\\ \Rightarrow \quad (a\quad -\quad 3)(a\quad -\quad 2)(a\quad +\quad 2)$