[tex]\\ \tt\hookrightarrow 12x^3+16x^2+21k+28[/tex]
[tex]\\ \tt\hookrightarrow 4x^2(3x+4)+7(3k+4)[/tex]
[tex]\\ \tt\hookrightarrow (4x^2+7)(3x+4)(3k+4)[/tex]
Done
