\(\left(\frac{3x}{7}+1\right):\left(-4\right)=\frac{-1}{28}\)
\(\Leftrightarrow\frac{3x}{7}+1=\frac{-1}{28}.\left(-4\right)\)
\(\Leftrightarrow\frac{3x}{7}+1=\frac{1}{7}\)
\(\Leftrightarrow\frac{3x}{7}=\frac{-6}{7}\)
\(\Leftrightarrow3x=-6\)
\(\Leftrightarrow x=-2\)