\documentclass[frontgrid,backgrid,letter,12pt]{flacards}

\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
        {$f(x) = \frac{x^2-4}{x+2}, x \in \mathbb{R}.$}
        {\includegraphics[width=.35\linewidth]{7.png}}
    
% Functions 3 & 10.
    \renewcommand{\frfoot}{3}
    \renewcommand{\brfoot}{10}
    \card
        {$f(x) = 
            \begin{cases}
                \text{undefined}, & x < 0 \\
                x^2, & x \ge 0
            \end{cases}.$}
        {\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
        {$f(x) = 
            \begin{cases}
                x+2, & x < 0 \\
                0, & x = 0 \\
                x+2, & x > 0
            \end{cases}.$}
        {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}}
        {$f(x) = 
            \begin{cases}
                x^3+6, & x < 0 \\
                x^3-6, & x \ge 0
            \end{cases}.$}

\end{document}