Question

$x {}^{2} - 4x - 21$

Answer 1

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

Answer 2

2nd identity used a

${a }^{2} - 2ab + {b}^{2}$

put up questions then solve your problem okjj(*_*)(^_^)