Question: How is it possible to sort equations align with their = like this:

```
a*b*c = d*e*f
  g*h = i*j
    k = l*m*n
o*p*q = r

```

I've seen it before but I don't know how to find that!

Answer: 
You're looking for the `align` environment which is provided by `amsmath`.

In the code below, an ampersand (&) denotes a vertical alignment point. Every line in the `align` environment will be aligned such that all ampersands are vertically aligned.

```
\documentclass{article}

\usepackage{amsmath}

\begin{document}

\begin{align}
    a*b*c &= d*e*f\\
    g*h &= i*j\\
    k &= l*m*n\\
    o*p*q &= r% \\ is not needed in this line!
\end{align}

\end{document}

```
If you need equation numbers, there are 2 options as follows.

An equation number for each line
--------------------------------

```
\documentclass[preview,border=10pt]{standalone}% change it back to
%\documentclass{article}

\usepackage{amsmath}

\begin{document}
\abovedisplayskip=0pt\relax% for my own purpose, remove this line!

\begin{align}
    a*b*c &= d*e*f\\
    g*h &= i*j\\
    k &= l*m*n\\
    o*p*q &= r% no \\ in this line!
\end{align}

\end{document}

```

![enter image description here](https://i.stack.imgur.com/wYIba.png)

An equation number for the whole as a group
-------------------------------------------

```
\documentclass[preview,border=10pt]{standalone}% change it back to
%\documentclass{article}

\usepackage{amsmath}

\begin{document}
\abovedisplayskip=0pt\relax% for my own purpose, remove this line!

\begin{equation}
\begin{split}
    a*b*c &= d*e*f\\
    g*h &= i*j\\
    k &= l*m*n\\
    o*p*q &= r% no \\ in this line!
\end{split}
\end{equation}

\end{document}

```

![enter image description here](https://i.stack.imgur.com/wjD5x.png)

But if you don't need equation numbers, replace `align` with `align*` or `equation` with `equation*`. `\[ ... \]` can also be used for the latter case!

Note: [`equation+split` versus `equation+aligned` can be found here](https://tex.stackexchange.com/a/109167/19356).