Help/Adv. Equations

Some numbered equations have multiple lines and generally

\begin{equation} \begin{align} \left( {{\rho }_{s}}-\rho \right)g\frac{\pi }{6}{{d}^{3}}={{C}_{D}}\frac{\pi }{8}\rho W_{s}^{2}{{d}^{2}} // \left( {{\rho }_{s}}-\rho \right)g\frac{\pi }{6}{{d}^{3}}={{C}_{D}}\frac{\pi }{8}\rho W_{s}^{2}{{d}^{2}} \end{align} \end{equation}

was typed as follows:
\ begin{equation}
   \left( {{\rho }_{s}}-\rho \right)g\frac{\pi }{6}{{d}^{3}}={{C}_{D}}\frac{\pi }{8}\rho W_{s}^{2}{{d}^{2}}
\ end{equation}