$$ \LARGE \begin{bmatrix} \color{red}a & \color{red}b & \color{red}c\\\color{black}d &\color{black} e &\color{black}f\end{bmatrix} \cdot \begin{bmatrix} \color{black}a' & \color{red}b'\\ \color{black}c' &\color{red} d' \\ \color{black}e' &\color{red} f'\end{bmatrix} = \LARGE \begin{bmatrix} \color{black}aa'+bc'+ce' & \color{red}ab'+bd'+cf'\\ \color{black}a'd+c'e+e'f &\color{black} b'd+d'e+f'f\end{bmatrix}$$

Got it !

Multiplication de deux matrices (1e dimension de la matrice 1 = 2e dimension de la matrice 2) :

$\LARGE \begin{bmatrix}1 & 2\\3 & 4\end{bmatrix} \cdot \begin{bmatrix}0 \\2 \end{bmatrix} = $	$\LARGE \begin{bmatrix}4\\8\end{bmatrix}$
$\LARGE \begin{bmatrix}1 & x & 3\\4 & y & 0\end{bmatrix} \cdot \begin{bmatrix}0 & -x \\1 & 1\\1 & 1\end{bmatrix} = $	$\LARGE \begin{bmatrix}x+3 & 3 \\y & y-4x \end{bmatrix}$