Question: I am trying to subtract one equation from another but I can't get it to display the way that I want. This is what I have right now.

```
\begin{align*}
&y&=2x+5\\   
-&&\\    
&y&=3x+10\\\hline   
&0&=-x-5    
\end{align*}

```

Answer: 
Here is my solution:
![enter image description here](https://i.stack.imgur.com/HLkh2.png)

```
\documentclass[]{article}

\usepackage{amsmath}
\begin{document}
\begin{alignat*}{4}
    &y&=&&2x&&+&5\\   
    -\hspace*{.5em}&y&=&&3x&&+&10\\[-2.5ex]\cline{2-8}\\[-4.8ex]
    &0&=&&-x&&-&5    
\end{alignat*}

\end{document}

```

I use the `alignat` enviroment from `amsmath`
Stealing the idea with `\frac` from [this answer](https://tex.stackexchange.com/a/137587/19356) but replacing the `matrix` with `aligned`. Is it a cute idea? I hope so!

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

\usepackage{amsmath}
\begin{document}
\abovedisplayskip=0pt\relax
\[
    \frac
    {
        \!\begin{aligned} 
                y &=    2x+5\\ 
                y   &=  3x+10 
        \end{aligned}
    }
    {0=-x-5}
    \ -
\]
\end{document}

```

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