# How to make a division line a bit longer than normal?

\documentclass[preview,border=12pt,12pt]{standalone}

\usepackage{mathtools}

\begin{document}
\displaystyle \begin{aligned} -\frac{3x}{10} &= 6 \\ -\frac{3}{10}x &= 6\\ x &= \frac{6}{-\tfrac{3}{10}}\\ &= -\frac{\tfrac{6}{1}}{\tfrac{3}{10}}\\ &= - \frac{6}{1}\times \frac{10}{3} \\ &= -20 \end{aligned}
\end{document}


# Result # Expected I don't think it's useful: the fraction line on the formula axis (midway of the = sign) is sufficient to tell the reader what's the main one.

Using \; is perhaps exaggerated, try with \,.

\documentclass{article}
\usepackage{mathtools}

\newcommand{\Mfrac}{% main fraction
\frac{\;#1\;}{\;#2\;}%
}

\begin{document}

\begin{aligned} -\frac{3x}{10} &= 6 \\ -\frac{3}{10}x &= 6\\ x &= \Mfrac{6}{-\tfrac{3}{10}}\\ &= -\Mfrac{\tfrac{6}{1}}{\tfrac{3}{10}}\\ &= - \Mfrac{6}{1}\times \Mfrac{10}{3} \\ % <-- is it good here? &= -20 \end{aligned}
\end{document} Perhaps you can manually add some white space by, say, ~, \, or even \hspace?

\begin{aligned} -\frac{3x}{10} &= 6 \\ -\frac{3}{10}x &= 6\\ x &= \frac{6}{-\tfrac{3}{10}}\\ &= -\frac{~\tfrac{6}{1}~}{\tfrac{3}{10}}\\ &= - \frac{~6~}{1}\times \frac{10}{3} \\ &= -20 \end{aligned} 