Align equations in cases around the equal sign

Question

So I'm trying to do a simple thing with cases:

$
\begin{cases}
-x+5y&=6\\
x-3y&=-4\\
\end{cases}
\\
\begin{cases}
-x+5y&=6\\
x&=3y-4\\
\end{cases}
$

And of course, LaTeX doesn't do what it should. For some reason, the "x" is far to the left.

How to make it look centered around the equal sign?

Answer 1

The cases environment is not meant for systems of equations as the name itself hints.

You want a proper alignment environment, most likely aligned:

\documentclass{article}
\usepackage{amsmath}

\begin{document}

\begin{align*}
&\left\{
  \begin{aligned}
  -x+5y&=6\\
  x-3y&=-4\\
  \end{aligned}
\right.
\\
&\left\{
  \begin{aligned}
  -x+5y&=6\\
  x&=3y-4\\
  \end{aligned}
\right.
\end{align*}

\end{document}

Answer 2

For the first ‘cases’, thez systeme package yields a nicer layout (and a simpler code). For the second “cases’, you can use the aligned environment inside cases:

\documentclass[12pt,a4paper]{article}

\usepackage{systeme}
\usepackage{mathtools}

\begin{document}

\noindent$
\systeme{-x+5y=6, x-3y=-4}
\\[1ex]
\begin{cases}
\begin{aligned}
-x+5y & =6,\\ x & =3y-4
\end{aligned}
\end{cases}
$

\end{document}