\documentclass{article}
\usepackage{tikz}
\usepackage{tikz-3dplot}
\usepackage[active,tightpage]{preview}
\PreviewEnvironment{tikzpicture}
\setlength\PreviewBorder{0.125pt}
%
% File name: heptahedron.tex
% Description: 
% A geometric representation of a heptahedron is shown.
% 
% Date of creation: June, 04th, 2021.
% Date of last modification: October, 9th, 2022.
% Author: Efraín Soto Apolinar.
% https://www.aprendematematicas.org.mx/author/efrain-soto-apolinar/instructing-courses/
% Source: page 224 of the 
% Glosario Ilustrado de Matem\'aticas Escolares.
% https://tinyurl.com/5udm2ufy
%
% Terms of use:
% According to TikZ.net
% https://creativecommons.org/licenses/by-nc-sa/4.0/
% Your commitment to the terms of use is greatly appreciated.
%
\begin{document}
\tdplotsetmaincoords{70}{110}
%
\begin{tikzpicture}[tdplot_main_coords]
	% Change the value of the number at {\escala}{##} to scale the figure up or down
	\pgfmathsetmacro{\escala}{1.0}
	\pgfmathsetmacro{\a}{3.0}
	% Coordinates of the vertices
	\coordinate(1) at (0,0,0);
	\coordinate(2) at (\a,0,0);
	\coordinate(3) at (\a,\a,0);
	\coordinate(4) at (0,\a,0);
	\coordinate(5) at (0,0,\a);
	\coordinate(6) at (\a,0,\a);
	\coordinate(7) at (\a,\a,\a);
	\coordinate(8) at (0,\a,\a);
	% Faces of the polyhedron
	\draw[red,thick,fill=cyan!35,opacity=0.75] (1) -- (2) -- (3) -- (4) -- cycle; % lower face
	\draw[red,thick,fill=cyan!35,opacity=0.75] (1) -- (2) -- (6) -- (5) -- cycle; % back face (on the plane xz)
	\draw[red,thick,fill=cyan!35,opacity=0.75] (1) -- (4) -- (8) -- (5) -- cycle; % back face (on the plane yz)
	\draw[red,thick,fill=cyan!35,opacity=0.75] (2) -- (3) -- (6) -- cycle; % front face (parallel to the plane yz)
	\draw[red,thick,fill=cyan!35,opacity=0.75] (3) -- (4) -- (8) -- cycle; % front face (parallel to the plane xz)
	\draw[red,thick,fill=cyan!35,opacity=0.75] (5) -- (6) -- (8) -- cycle; % top face
	\draw[red,thick,fill=cyan!35,opacity=0.75] (6) -- (3) -- (8) -- cycle; % slanted face (inclined plane)
	%
	\end{tikzpicture}
	%
\end{document}



