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