$$(x-9)(x+1) \geq 0$$
$x\in (-\infty,-1]\cup [9,\infty)$
$$x+1\leq 0$$ $$x-9\leq 0$$
$$x\leq -1$$
$$x-9\geq 0$$ $$x+1\geq 0$$
$$x\geq 9$$
$$x\leq -1\text{; }x\geq 9$$
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