## Elementary and Intermediate Algebra: Concepts & Applications (6th Edition)

$(x-5)(x+4)$
The given expression is equivalent to \begin{array}{l}\require{cancel} x^2-20-x \\\\= x^2-x-20 .\end{array} The trinomial expression above in the form $ax^2+bx+c,$ has $c$ equal to $-20$ and $b$ equal to $-1 .$ The two numbers whose product is $c$ and whose sum is $b$ are $\{ -5,4 \}.$ Hence, the factored form of the expression above is \begin{array}{l}\require{cancel} (x-5)(x+4) .\end{array}