I would like to write an equation consisting of one equation with matrices and one equation referencing the above matrices underneath (see picture).
What I tried can be seen in a minimized version in the code. It throws error 'Extra alignment tab has been changed to \cr'. When i skip the error multiple times every column stands in a own row.
\begin{equation}\label{eq:NewEul}\begin{split}
\begin{bmatrix} M1 & & 0 \\
& \ddots & \\
0 & & Mn \end{bmatrix}
& \begin{bmatrix} a1 \\
\vdots \\
an \end{bmatrix}
& =
& \begin{bmatrix} f1 \\
\vdots \\
fn \end{bmatrix} \\
M & a & = & f
\end{split}\end{equation}
My question is: How do I need to modify my code regarding the alignment to match the style shown in the picture?