Given the expression
[tex]9x^2-120x-400[/tex]9 is a perfect square of
[tex]9=(3)^2[/tex]400 is also a perfect square of
[tex]400=(20)^2[/tex]Both the x-term and the constant are negative, which indicates that this polynomial is the result of a difference of squares:
[tex](a+b)(a-b)=a^2-ab+ab-b^2=a^2-b^2[/tex]So the original factorization is
[tex](3x+20)(3x-20)[/tex]