\documentclass{article}

\usepackage{amsmath,amsfonts,graphicx}
%
%\renewcommand{\cardtextstylef}{\center}
%\renewcommand{\cardtextstyleb}{\center}
%\renewcommand{\flhead}{Expert-Group Role}
%\renewcommand{\blhead}{Home-Group Role}
%Needed for my printer.
%\addtolength{\topmargin}{-0.3cm}
%\addtolength{\evensidemargin}{-1mm}

%\pagesetup{2}{3}

\begin{document}

%% Functions 1 & 8.
%    \renewcommand{\frfoot}{1}
%    \renewcommand{\brfoot}{8}    
%    \card
%        {$f(x) = 
%            \begin{cases}
%                x, & x < 0 \\
%                0, & x \ge 0
%            \end{cases}.$}
%        {\includegraphics[width=.35\linewidth]{8.png}}
%    
% Functions 2 & 7.
%    \renewcommand{\frfoot}{2}
%    \renewcommand{\brfoot}{7}
%    \card
%        {
%Card 2:
%
%$$f(x) = \frac{x^2-4}{x+2}, x \in \mathbb{R}.$$
%}
%        {\includegraphics[width=.35\linewidth]{7.png}}
%    
%% Functions 3 & 10.
%    \renewcommand{\frfoot}{3}
%    \renewcommand{\brfoot}{10}
%    \card
%        {
%Card 3:
%
%$$f(x) = 
%            \begin{cases}
%                \text{undefined}, & x < 0 \\
%                x^2, & x \ge 0
%            \end{cases}.$$
%}
%        {\includegraphics[width=.35\linewidth]{10.png}}
%
%% Functions 4 & 9.
%    \renewcommand{\frfoot}{4}
%    \renewcommand{\brfoot}{9}
%    \card
%        {\includegraphics[width=.35\linewidth]{4.png}}
%        {\includegraphics[width=.35\linewidth]{9.png}}
%
%
%% Functions 5 & 12.
%    \renewcommand{\frfoot}{5}
%    \renewcommand{\brfoot}{12}    
%    \card
%        {$f(x) = 
%            \begin{cases}
%                x+2, & x < 0 \\
%                0, & x = 0 \\
%                x+2, & x > 0
%            \end{cases}.$}
%        {
Card 12:

Heaviside's theta function: \\ \vspace{12pt} $$f(x) = 
            \begin{cases}
                0, & x < 0 \\
                1, & x > 0
            \end{cases}.$$
%}
%    
%% Functions 6 & 11.
%    \renewcommand{\frfoot}{6}
%    \renewcommand{\brfoot}{11}    
%    \card
%        {\includegraphics[width=.35\linewidth]{6.png}}
%        {
%Card 11:
%
%$$f(x) = 
%            \begin{cases}
%                x^3+6, & x < 0 \\
%                x^3-6, & x \ge 0
%            \end{cases}.$$
%            }

\end{document}