Funes el memorioso

\documentclass{standalone}

\usepackage{tikz}

\begin{document}
  \begin{tikzpicture}
    \tikzstyle{abox} = [rectangle, thick, text centered ,
    text width=3.5cm, minimum height=1cm,
    top color=white, bottom color=blue!30, draw=blue!40]
    \tikzstyle{xbox} = [rectangle, thick, text centered ,
    text width=.3cm, minimum height=3.5cm,
    top color=white, bottom color=red!30, draw=red!40]
    \tikzstyle{bbox} = [rectangle, thick, text centered ,
    text width=.3cm, minimum height=1cm,
    top color=white, bottom color=green!30, draw=green!40]
    
    \coordinate (A) at (0cm, 0cm);
    \coordinate (X) at (3cm, 0cm);
    \coordinate (E) at (4cm, 0cm);
    \coordinate (B) at (5.2cm, 0cm);

    % measurement
    \node [abox] (ABox) at (A) {$\mathbf{A}$};
    \node [left]  at (ABox.west)  {$\scriptstyle m$};
    \node [below] at (ABox.south) {$\scriptstyle n$};
    \node [yshift=-2.5cm] at (A) {measurement};

    % input
    \node [xbox] (XBox) at (X) {$\mathbf{x}$};
    \node [left]  at (XBox.west)  {$\scriptstyle n$};
    \node [below] at (XBox.south) {$\scriptstyle 1$};
    \node [yshift=-2.5cm] at (X) {input};

    % equal
    \node at (E) {=};

    % observation
    \node [bbox] (BBox) at (B) {$\mathbf{b}$};
    \node [left]  at (BBox.west)  {$\scriptstyle m$};
    \node [below] at (BBox.south) {$\scriptstyle 1$};
    \node [yshift=-2.5cm] at (B) {observation};
  \end{tikzpicture}
\end{document}