# Alignment spacing for negatives - Why am I unable to properly align my equations?

Although I searched the Web and indeed found an answer here, I still seem unable to address my issue of aligning two rows in my step-by-step process for solving a very basic equation. Just in case it matters, I am using MathJax, which is a JavaScript library that renders LaTeX in various forms of output (in my case, as MathML and HTML/CSS).

My code:

\begin{eqnarray}
\hphantom{-} 9 + x &=& 12   \\
-9 \hphantom{+ x} &=& -9    \\
\hline                      \\
\implies x &=& 3
\end{eqnarray}


My goal is to get the "9" to align in the first two rows, however, the \hphantom macro in the first row isn't adjusting the 9 + x = 12 at all. I came up with the code using the help of people who answered a similar alignment question elsewhere here.

I noted that when I reversed the first two rows in my code, \hphantom did indeed align the 9 correctly.

I would appreciate anyone helping me determine if there is an issue with my LaTeX. If not, I can investigate the HTML/CSS layout issues.

-
Yes, it matters because MathJaX is not really TeX, it only uses TeX for markup. Though, you can try \hphantom{{} + x}. And do not use eqnarray: eqnarray vs align –  Qrrbrbirlbel Mar 3 '13 at 6:29
Thank you, @Qrrbrbirlbel, your suggestion addressed the problem perfectly. I'll better keep in mind that MathJax may have discrepancies down the line, as well. EDIT: also, I wasn't aware of the issues of eqnarray, thank you for the reference! –  Curtis Mar 3 '13 at 7:25
Welcome to TeX.sx! –  texenthusiast Mar 3 '13 at 12:18
Summarize your findings (and the comments) into an answer, and later accept it to close the issue. –  vonbrand Mar 3 '13 at 17:46

Thanks to @Qrrbrbirlbel, I was able to address the problem by using: \hphantom{{} + x}. So, if you're a MathJax user, this does address the issue.
The spacing rules are in the TeXbook. + is class \mathbin when between to ordinary (\mathord) symbols such as x, 9 or {}, otherwise it is an ordinary symbol. No spacing is placed between mathord's, whereas Ord-Bin-Ord gets extra spacing around the binary operator. The reason that a unitary + should be placed right up by the symbol it affects, whereas a binary one should be spaced off from its symbols. –  Andrew Swann Mar 11 '13 at 8:04