\documentclass[12pt,twoside]{article}
\usepackage{amssymb}
\usepackage[fleqn]{amsmath}
\begin{document}



\textbf{eqnarray}:
\begin{eqnarray}
  p(x) \;=&\; a_n x^n \,+\,a_{n-1} x^{n-1} \,+ \,+\,\hdots\,+\,a_2 x^2 \,+\, a_1 x \,+\,a_0 + \nonumber \\
	      & b_m y^m \,+\,b_{m-1} y^{m-1} \,+ \,+\,\hdots\,+\,b_2 y^2 \,+\, b_1 y \,+\,b_0
\end{eqnarray}

\bigskip
\textbf{align}:
\begin{align}
  p(x) \;=&\; a_n x^n \,+\,a_{n-1} x^{n-1} \,+ \,+\,\hdots\,+\,a_2 x^2 \,+\, a_1 x \,+\,a_0 + \nonumber \\
	      & b_m y^m \,+\,b_{m-1} y^{m-1} \,+ \,+\,\hdots\,+\,b_2 y^2 \,+\, b_1 y \,+\,b_0
\end{align}

\bigskip
\textbf{multline}:
\begin{multline}
  p(x) \;=\; a_n x^n \,+\,a_{n-1} x^{n-1} \,+ \,+\,\hdots\,+\,a_2 x^2 \,+\, a_1 x \,+\,a_0 +\\
	       b_m y^m \,+\,b_{m-1} y^{m-1} \,+ \,+\,\hdots\,+\,b_2 y^2 \,+\, b_1 y \,+\,b_0
\end{multline}

\bigskip
\textbf{split}:
\begin{equation}
    \begin{split}
        p(x) \;=&\; a_n x^n \,+\,a_{n-1} x^{n-1} \,+ \,+\,\hdots\,+\,a_2 x^2 \,+\, a_1 x \,+\,a_0+ \\
	      & b_m y^m \,+\,b_{m-1} y^{m-1} \,+ \,+\,\hdots\,+\,b_2 y^2 \,+\, b_1 y \,+\,b_0
    \end{split}
\end{equation}


\end{document}