# Equations tag placement: In between two equations

I would like to use only one tag (O.C.) for these two equations and place it on the right side as by default, but, (vertically), in the middle between the two equations.

Is this possible? Is this right according to LaTeX rules? \documentclass[12pt, a4]{article}
\usepackage{amsmath,amsfonts,amssymb}

\begin{enumerate}[a.]
\item \textit{Solve for the equilibrium prices of the contingent claims and the resulting consumption amounts.} Knowing already the first derivative of the two utility functions, we set the \textit{Optimally Condition}
\begin{gather}
\tag{O.C.} \frac{0.5}{\Pi_{NL}} \cdot \frac{1}{C^A_{NL}} = \frac{0.5}{\Pi_{L}} \cdot \frac{1}{C^A_{L}} \quad \to \quad \Pi_{NL}\cdot C^A_{NL} = \Pi_{L} \cdot C^A_L \\
\tag{O.C.} \frac{0.5}{\Pi_{NL}} \cdot \frac{1}{C^B_{NL}} = \frac{0.5}{\Pi_{L}} \cdot \frac{1}{C^B_{L}} \quad \to \quad \Pi_{NL}\cdot C^B_{NL} = \Pi_{L} \cdot C^B_L
\end{gather}
\end{enumerate}
\end{document}

• Aside: For correct English, the expression should be \textit{Optimality Condition}, not \textit{Optimally Condition}. (Optimally is an adverb, optimality is a noun.) – Mico Oct 25 '16 at 3:41

There are several options to achieve this. One possibility is to use the \begin{split} .. \end{split} environment inside the equation. Another option is to use \begin{aligned} .. \end{aligned}.

Note: Paper size of A4 is declared with a4paper option and not a4, also you need the enumerate package to customize enumerate items.

\documentclass[12pt,a4paper]{article}
\usepackage{amsmath,amsfonts,amssymb,enumerate}
\begin{document}
\begin{enumerate}[a.]
\item \textit{Solve for the equilibrium prices of the contingent claims and the resulting consumption amounts.} Knowing already the first derivative of the two utility functions, we set the \textit{Optimally Condition}
\begin{equation}\tag{O.C.}
\begin{split}
\frac{0.5}{\Pi_{NL}} \cdot \frac{1}{C^A_{NL}} &= \frac{0.5}{\Pi_{L}} \cdot \frac{1}{C^A_{L}} \quad \to \quad \Pi_{NL}\cdot C^A_{NL} = \Pi_{L} \cdot C^A_L \\
\frac{0.5}{\Pi_{NL}} \cdot \frac{1}{C^B_{NL}} &= \frac{0.5}{\Pi_{L}} \cdot \frac{1}{C^B_{L}} \quad \to \quad \Pi_{NL}\cdot C^B_{NL} = \Pi_{L} \cdot C^B_L
\end{split}
\end{equation}
\end{enumerate}
\end{document} 