Answer
$-(x-4)(x+5)$
Work Step by Step
We start by factoring out -1. We have $-x^2-x+20=-(x^2+x-20)$
The factor pairs of -20 are
1 and -20, -1 and 20
2 and -10, -2 and 10
4 and -5, -4 and 5
-4 and 5 add up to 1.
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