[Tex/LaTex] How to break a long equation

Question

Upon compiling, one of my equations extends out of my column width in my final output. Options I tried–

1. breqn package asks for expl3.sty file which the compiler is unable to locate, (would have been the easiest way in the {dmath} environment
2. // in the equation,
3. {split}, {multiline} environment of amsmath but none of the 3 approaches seem to work.

Any tip from your Texperience(!)?

PS: I use latest version of TexnicCenter (+MikTeX 2.9) and the equation is in the {equation} environment right now.

Answer 1

none of the 3 approaches seem to work

That is a bad sentence. Do you get any errors?

Here is an example of the documentation of amsmath with split

\documentclass{article}
\usepackage{amsmath}
\begin{document}

\begin{equation}\label{e:barwq}
\begin{split}
 H_c&=\frac{1}{2n} \sum^n_{l=0}(-1)^{l}(n-{l})^{p-2}
     \sum_{l _1+\dots+ l _p=l}\prod^p_{i=1} \binom{n_i}{l _i}\\
    &\quad\cdot[(n-l )-(n_i-l _i)]^{n_i-l _i}\cdot
      \Bigl[(n-l )^2-\sum^p_{j=1}(n_i-l _i)^2\Bigr].
\end{split}
\end{equation}

\end{document}