\documentclass{article}


\usepackage{amsmath}

\begin{document}

\begin{equation}\label{eq:left}
  \begin{split}
    100 &= 1+8+27+64 = {}\\
        &= 1+3+5+7+9+{}\\
        &\quad+11+13+15+17+19
  \end{split}
\end{equation}

\end{document}