$(x^{2} - 4 x - 21)$ =?

(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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