How to fit a large equation that goes out of the page?

Mathematical symbols where created to represent the meaning of longer descriptive words in a short form for easy inclusion in complex formulas with possible repetition. I would choose a suitable symbol/character to represent that long sentence like this (choose your own descriptive symbol):

\documentclass{article}
\begin{document}

\begin{equation}
  Ep = P_l \times da_i,
\end{equation}
%
where $P_l =$ Required Personality Level of Each Resource-Assigned Personality Level of all Resources

\end{document}

break your long text into two lines:

\documentclass{article}

\begin{document}
\begin{equation}
    Ep = \left(\begin{tabular}{l}
                Required Personality Level of Each Resource-\\
                Assigned Personality Level of each Resources
                \end{tabular}\right)
        \times (da_{i})
\end{equation}
\end{document}