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: 
My two-cents worth, using more of a textual representation and an `array`:

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

```
\documentclass{article}
\usepackage{amsmath}% http://ctan.org/pkg/amsmath
\begin{document}
\[
  \begin{array}{l@{\quad}cr@{}l}
    && y & {}= 2x+5 \\
    \text{minus} && y & {}= 3x+10 \\ \cline{2-4}
    && 0 & {}= -x-5
  \end{array}
\]
\end{document}​

```

The additional (second) column is to allow for the `\cline` to have a little padding around the equations. Modify `\quad` to `\hspace{<len>}` to have a gap of length `<len>` between "minus" and the equation.
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)