\begin{tikzpicture}[scale = 0.6, every node/.style={scale=0.8}]
    \draw[
        fill,
        lightgray
    ] (-2,0) -- (3,0) -- (-0.5,-6) -- (-2,-6) -- cycle;


    \draw[
        fill,
        red
    ] (-2, -3) -- (-2,-4) -- (2,-6) -- (4.3,-6) -- (4.3,-4) -- cycle;

    \draw[
        -to,
        line width = 1.5pt,
    ] (3,-1) -- (4.3,-1);

    \draw[
        fill,
        blue
    ]  (1.7,-2) circle (15pt);

    \draw[
        fill,
        blue
    ]  (0.4,-4.2) circle (15pt);

    \node (fu) at (1.4,-1) {\LARGE $f_u$ $(+)$};

    \node (fl) at (2.4,-3) {\LARGE $f_l$ $(-)$};

    \node (nocontrin) at (2,-4.5) {\LARGE $f_u=f_l$};

\end{tikzpicture}