\documentclass{article}
\usepackage{amsmath}
\usepackage{mathtools}

\begin{document}


\begin{equation}\label{eq:elas:C}
\end{equation}

\begin{equation}
  \mu     = \frac{E    }{2(1+\nu)        }
\end{equation}

\begin{align}
  a & = b + c^2 \\
  c & =
      \begin{multlined}[t]
        d + e + f + g + h + i + j \\
        + l + k + m + n + o + p + q
      \end{multlined}
\end{align}

\end{document}

