[tex]\\ \rm\rightarrowtail x^2-21x+108[/tex]
[tex]\\ \rm\rightarrowtail x^2-12x-9x+108[/tex]
[tex]\\ \rm\rightarrowtail x(x-12)-9(x-12)[/tex]
[tex]\\ \rm\rightarrowtail (x-9)(x-12)[/tex]
Done!
