I'm sorry I don't exactly know how to word the question, a figure should show what I want.
Here's what I have
[![enter image description here](https://i.stack.imgur.com/A7hnJ.png)](https://i.stack.imgur.com/A7hnJ.png)

And here's what I want
[![enter image description here](https://i.stack.imgur.com/8oRYe.png)](https://i.stack.imgur.com/8oRYe.png)

M\_f is a matrix whose components are not determined so I just want to write it's name inside the system.

Here's the code

```
\documentclass[a4paper,12pt,twoside,openright]{report}
\usepackage{amsmath}
\usepackage{arydshln}

\begin{document}

\begin{equation}
\renewcommand{\arraystretch}{1.4}
\left[
\begin{array}{c;{2pt/2pt} c c}
 M_s &  & \\ \hdashline[2pt/2pt]
 & \matr{M}_f & \\
-\rho &  &
\end{array}
\right] \left[
\begin{array}{c}
\ddot{u} \\ \hdashline[2pt/2pt]
p_1 \\
p_2
\end{array}
\right]
\end{equation}

\end{document}

```

Thank you for your help.

And also if someone can advice me how to change the thickness of the dashed lines in the matrix, I tried changing the pt but what happens is that only the thickness change and not the the width. I can't post a third image since I only have 10 reputation points to show you the difference.