21 номер, пожалуйста

$\\ \frac{-18}{x^2 - 4x - 21} \leq 0 \\ \\ x^2 - 4x - 21 = 0 \\ \\ D = (-4)^2 - 4*(-21) = 16 + 84 = 100 \\ \\ \sqrt{100}=10 \\$
$\\ x_1 = \frac{4+10}{2} = \frac{14}{2} = 7 \\ \\ x_2 = \frac{4-10}{2} = -3 \\$
$\\ x^2 - 4x - 21 = (x-7)(x+3) \\ \\ \frac{18}{(x-7)(x+3)} \leq 0 \\ \\ - 18(x-7)(x+3) \leq 0 \\ \\ (x-7)(x+3) \geq 0\\ \\x_1 = 7 \\ x_2 = -3 \\$
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x∈(-∞;-3) U (7 ; + ∞)