\(y+70y+70\%y=-5,1\)
\(\Leftrightarrow y\left(1+70+70\%\right)=-5,1\)
\(\Leftrightarrow y\left(1+70+\frac{70}{100}\right)=-5,1\)
\(\Leftrightarrow y\left(71+\frac{70}{100}\right)=-5,1\)
\(\Leftrightarrow y\left(\frac{710}{10}+\frac{7}{10}\right)=-5,1\)
\(\Leftrightarrow\frac{717}{10}x=-5,1\)
\(\Leftrightarrow x=-5,1\div\frac{717}{10}\)
\(\Leftrightarrow x=-5,1\times\frac{10}{717}\)
\(\Leftrightarrow x=\frac{-51}{717}\)