# (x+7)(x+9)=(x+3)(x+21)

$\left(x+7\right)\left(x+9\right)=\left(x+3\right)\left(x+21\right)\phantom{\rule{0ex}{0ex}}{x}^{2}+9x+7x+63={x}^{2}+21x+3x+63\phantom{\rule{0ex}{0ex}}16x=24x\phantom{\rule{0ex}{0ex}}8x\left(3-2\right)=0\phantom{\rule{0ex}{0ex}}x=0$

Regards

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