Numbering an array as one single equation LaTeX

I would recommend you use the equation environment if you want a single equation for an entire array.

However, since this construct does not really seem to need an array, I would recommend using the align environment from the amsmath package, with which you can use \nonumber to selectively disable an equation number.

Notes:

• Use of eqnarray is not recommended as per \eqnarray vs \align.

• Also you should not use double dollar signs for display math as per Why is \[ … \] preferable to $$?

• The math spacing in you array example was not correct. This has been corrected in the example below.

References:

• An excellent reference for math mode is Herbert Voss' comprehensive review of mathematics in (La)TeX.

Code:

\documentclass{article}
\usepackage{amsmath}

\begin{document}\noindent
Using \verb|equation| with \verb|array|:
\begin{equation}
\begin{array}{r@{}l}
    N_{x,sk} &{}= k_{sk}\left(\frac{t_{sk}}{b_{sk}}\right)^{2}\bar{Et}\\
    N_{x,st} &{}= k_{st}\left(\frac{t_{st}}{b_{st}}\right)^{2}\bar{Et}
\end{array}
\end{equation}
%
Using \verb|align|:
\begin{align}
    N_{x,sk} &= k_{sk}\left(\frac{t_{sk}}{b_{sk}}\right)^{2}\bar{Et}\\
    N_{x,st} &= k_{st}\left(\frac{t_{st}}{b_{st}}\right)^{2}\bar{Et}
\end{align}
%
Using \verb|align| with \verb|\nonumber|:
\begin{align}
    N_{x,sk} &= k_{sk}\left(\frac{t_{sk}}{b_{sk}}\right)^{2}\bar{Et}\nonumber\\
    N_{x,st} &= k_{st}\left(\frac{t_{st}}{b_{st}}\right)^{2}\bar{Et}
\end{align}
\end{document}

Using the eqnarray environment instead of array, you can add \nonumber to any lines you don't want numbered. If you don't do this, every line will be numbered.

\begin{eqnarray}
N_{x,sk} &=& k_{sk}\left(\frac{t_{sk}}{b_{sk}}\right)^{2}\bar{Et} \nonumber \\
N_{x,st} &=& k_{st}\left(\frac{t_{st}}{b_{st}}\right)^{2}\bar{Et}
\end{eqnarray}

(Just remove the \nonumber from the above to get the second example.)