---Multiply a x c
Quadratic Form: ax^2 + bx + c
4 x -25 = -100
---Find a factor pair of -100 that adds up to -15
-20 x 5 = - 100
-20 + 5 = -15
---Separate the b term into your two factors
4n^2 + 5n - 20n - 25
This can take some trial and error (though there are only two options) because when you factor, you want the inside of the parentheses to be the same. If you look at the next step, (4n + 5) appears twice.
---Factor each binomial
n(4n + 5) - 5 (4n + 5)
---Rewrite
(n - 5)(4n + 5)
What you're really doing here is factoring out the (4n + 5), which leaves you with the terms that you factored out in the previous step. You then combine these terms into one set of parenthesis.
Answer: (n - 5)(4n + 5)
Hope this helps!