\(\frac{-7}{x}=\frac{-21}{\left(x-34\right)}\)
\(\Leftrightarrow\frac{\left(x-34\right)}{-21}=\frac{x}{-7}\)
\(\Leftrightarrow\frac{\left(x-34\right)}{-21}=\frac{x}{-7}\)
\(\Leftrightarrow\frac{\left(x-34\right)}{-21}=\frac{3x}{-21}\)
\(\Leftrightarrow x-34=3x\)
\(\Leftrightarrow x-3x=34\)
\(\Leftrightarrow x\left(1-3\right)=34\)
\(\Leftrightarrow x\left(-2\right)=34\)
\(\Leftrightarrow x=34:\left(-2\right)=-17\)