#### Answer

$-1$

#### Work Step by Step

Factoring the expressions and then cancelling the common factor/s between the numerator and the denominator, the given expression, $
\dfrac{x-12}{12-x}
,$ simplifies to
\begin{array}{l}\require{cancel}
\dfrac{-1(-x+12)}{12-x}
\\\\=
\dfrac{-1(12-x)}{12-x}
\\\\=
\dfrac{-1(\cancel{12-x})}{\cancel{12-x}}
\\\\=
-1
.\end{array}