Hope you don't mind a followup! :-) Is there a way to right justify the equation numbers?
(I'm guessing the items in red are standard LaTeX macros I can find documented somewhere?)
yes, but it is easier to write the _whole_ stuff in real LaTeX code, LyX is not designed for such speical stuff.
Attached a LyX solution,using tabularx
Herbert
#LyX 1.3 created this file. For more info see http://www.lyx.org/ \lyxformat 221 \textclass article \begin_preamble \usepackage{tabularx} \end_preamble \language frenchb \inputencoding latin1 \fontscheme default \graphics default \paperfontsize 12 \spacing single \papersize Default \paperpackage a4 \use_geometry 0 \use_amsmath 1 \use_natbib 0 \use_numerical_citations 0 \paperorientation portrait \paperwidth 17cm \paperheight 22cm \leftmargin 1.2cm \topmargin 1.4cm \rightmargin 1.7cm \bottommargin 1.4cm \headsep 1cm \secnumdepth 4 \tocdepth 4 \paragraph_separation indent \defskip medskip \quotes_language german \quotes_times 2 \papercolumns 1 \papersides 2 \paperpagestyle default \bullet 1 2 3 -1 \end_bullet
\layout Standard \begin_inset Formula \[ \left\{ \begin{array}{rrrrr} (3-\lambda)C & + & 2D & = & 0\qquad\refstepcounter{equation}(\theequation)\\ C & + & (4-\lambda)D & = & 0\qquad\refstepcounter{equation}(\theequation)\end{array}\right.\] \end_inset \layout Standard next try \layout Standard \begin_inset Formula \[ \left\{ \begin{array}{rrrrr} (3-\lambda)C & + & 2D & = & 0\rlap{\hspace{3cm}\refstepcounter{equation}(\theequation)}\\ C & + & (4-\lambda)D & = & 0\rlap{\hspace{3cm}\refstepcounter{equation}(\theequation)}\end{array}\right.\] \end_inset \layout Standard and the same with amsmath and cases \layout Standard \begin_inset Formula \[ \begin{cases} (3-\lambda)C+2D & =0\rlap{\hspace{3cm}\refstepcounter{equation}(\theequation)}\\ \hphantom{(3-\lambda)}C+(4-\lambda)D & =0\rlap{\hspace{3cm}\refstepcounter{equation}(\theequation)}\end{cases}\] \end_inset \layout Standard \begin_inset ERT status Collapsed \layout Standard \backslash noindent \backslash begin{tabularx}{ \backslash linewidth}{lXr} \newline \backslash hspace*{3cm} & \end_inset \begin_inset Formula $\begin{cases} (3-\lambda)C+2D & =0\\ \hphantom{(3-\lambda)}C+(4-\lambda)D & =0\end{cases}$ \end_inset \begin_inset ERT status Collapsed \layout Standard & $ \backslash begin{array}{c} \backslash refstepcounter{equation}( \backslash theequation) \backslash \backslash \newline \backslash refstepcounter{equation}( \backslash theequation) \backslash end{array}$ \end_inset \newline \begin_inset ERT status Collapsed \layout Standard \backslash end{tabularx} \end_inset \the_end