Error uploading to Moodle when using the matrix environment

The manual for moodle.sty (page 10) mentions that only four environments are recognized and converted to html: center, enumerate, itemize and tikzpicture. All other environments, such as matrix, therefore cannot be used. Quote (emphasis mine):

Be aware that moodle.sty does not know how to convert any other TEX or LATEX commands to HTML. If other sequences are used, they may be passed verbatim to the XML file or may cause unpredicted results.

However, within a tikzpicture you can use LaTeX code inside of a node, which allows you to use the matrix environment. The matrices will be converted to images by Moodle and stored in base64 encoding in the .xml file, similar to including .png files for example.

Note that to make this work you may need to call tikzexternalize explicitly in your quiz source code. Also shell-escape is required (same as with images).

MWE:

documentclass[12pt]{article}
usepackage{amsmath,amsthm, amssymb, latexsym}
usepackage{tikz}
makeatletter
renewcommand*env@matrix[1][c]{hskip -arraycolsep
let@ifnextcharnew@ifnextchar
array{*c@MaxMatrixCols #1}}
makeatother 
usepackage{moodle}
usetikzlibrary{external} % set this 
tikzexternalize          % explicitly

begin{document}
begin{quiz}{Revisiting Linear Algebra}
begin{multi}[points=2]{Matrix Form of a Linear System}
Consider the following system
       
begin{tikzpicture} % matrix inside of tikzpicture node
node{
$begin{matrix}[r]
3.0 x_1    &+2.0 x_2     &+2.0  x_3      & -5.0 x_4 & =8 .0   
0.6 x_1   &+ 1.5 x_2     &+1.5 x_3       & -5.4 x_4 & =2.7   
1.2 x_1   & -0.3 x_2      & -0.3 x_3      & 2.4 x_4   &  =2.1 ,   
end{matrix}
$};
end{tikzpicture}

which may be written as a single vector equation; i.e, $mathbf{AB}=mathbf{B}$.
The $mathbf{A}$ matrix associated with the system is:
item*
begin{tikzpicture}
node{ $
begin{bmatrix}[r]
3.0    &+2.0      &+2.0        & -5.0   
0.6    &+ 1.5      &+1.5        & -5.4     
1.2    & -0.3      & -0.3      & 2.4 ,   
end{bmatrix}
$};end{tikzpicture}
itembegin{tikzpicture}
node{ $
begin{bmatrix}[r]
8 .0   
2.7   
2.1    
end{bmatrix}
$};end{tikzpicture}  
  item begin{tikzpicture}
node{$
begin{bmatrix}[r]
x_1 &
x_2&   
x_3 &  
x_4 &
end{bmatrix}
$};end{tikzpicture}       
item  
begin{tikzpicture}
node{$
begin{bmatrix}[r]
3.0    &+2.0      &+2.0        & -5.0 &  8.0 
0.6    &+ 1.5     &+1.5       & -5.4  & 2.7   
1.2    & -0.3      & -0.3       & 2.4 , & 2.1  
end{bmatrix}
$};end{tikzpicture}       
end{multi}
 end{quiz}
end{document}

Part of the resulting xml:

<questiontext format="html">
    <text><![CDATA[<p>Consider the following system </P>
<P><IMG SRC="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAc4AAACOCAYAAAC4 etc.

The best practise solution seems to replace matrix with array one as proposed by the Moodle manual, as follows:

documentclass[12pt]{article}
usepackage{amsmath}
usepackage{moodle}
   begin{document}
   begin{quiz}{Revisiting Linear Algebra}
    
     begin{multi}[points=2]{Matrix Form of a Linear System}
       Consider the following system
       [
        left[
begin{array}{ccccc}
3.0 x_1    &+2.0 x_2     &+2.0  x_3      & -5.0 x_4 & =8.0
0.6 x_1   &+ 1.5 x_2     &+1.5 x_3       & -5.4 x_4 & =2.7  
1.2 x_1   & -0.3 x_2      & -0.3 x_3      & 2.4 x_4   &  =2.1 ,   
end{array}right]
]
  
which may be written as a single vector equation; i.e, $mathbf{AB}=mathbf{B}$.
     The $mathbf{A}$ matrix associated with the  system is:   
item* First answer
[
 left[
begin{array}{cccc}
3.0    &+2.0      &+2.0        & -5.0   
0.6    &+ 1.5      &+1.5        & -5.4     
1.2    & -0.3      & -0.3      & 2.4 , 
end{array}
right]
]
item Second answer
end{multi}

 end{quiz}
end{document}

Important note: the currently published moodle package version 0.5 is outdated and has a bug that does not handle . You have to download the latest version from here. Installation guidelines are here.