To factorize this expression, we start by "looking" for two terms whose sum will give "-n" and whose product will give "-20n-squared"
[tex]\begin{gathered} n^2\text{ - n - 20} \\ \text{This terms are (-5n and +4n)} \\ \text{Next, we replace the middle term (-n) with these new terms } \\ n^2\text{ -5n + 4n - 20} \\ n(n\text{ - 5) + 4(n - 5)} \\ (n\text{ + 4)(n - 5)} \end{gathered}[/tex]