Answer
$$(x - 5)(x + 4)$$
Work Step by Step
To factor this trinomial, you have to look at the last term and the middle term. You want to find the factors of the last term that, when added together, will give you the coefficient of the middle term.
Let us look at the factors for the last, or constant, term, which is $-20$, in this case:
$-20$ and $1$ or $20$ and $-1$
$-10$ and $2$ or $10$ and $-2$
$-5$ and $4$ or $-4$ and $5$
It looks like $-5$ and $4$ will add up to the coefficient of the middle term, which is $-1$, in this case.
Let us now do our factorization of the trinomial:
$$(x - 5)(x + 4)$$
