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.
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`