(x2 − 4x − 21) = ?
(a) (x − 7)(x − 3)
(b) (x + 7)(x − 3)
(c) (x − 7)(x + 3)
(d) none of these

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Solution

(c) (x − 7)(x + 3)

$x^{2} - 4 x - 21 = x^{2} - 7 x + 3 x - 21$
$= x (x - 7) + 3 (x - 7) = (x - 7) (x + 3)$

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