Answer
$-\frac{1}{3}$
Work Step by Step
$\frac{7x-21}{63-21x}=\frac{7(x-3)}{21(3-x)}=\frac{7(x-3)}{-21(x-3)}=-\frac{7}{21}\times\frac{x-3}{x-3}=-\frac{1}{3}\times1=-\frac{1}{3}$
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