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
