6. It is known that \(a b=c k \neq 0\). Simplify the fraction:

\[
\frac{a-b-c+k}{a+b+c+k}
\]
6. Solution. By multiplying the numerator and the denominator of the fraction by \(\boldsymbol{c} \neq 0\), performing the substitution, and using the equality \(a b=c k\), we get:

\[
\frac{a c-b c-c^{2}+a b}{a c+b c+c^{2}+a b}=\ldots=\frac{a-c}{a+c}
\]

Alternatively, from the given equality, we can express, for example, \(k\), perform the substitution, and simplify the resulting expression.