\documentclass[12pt]{article}
\usepackage{amsmath, amssymb, graphicx, array, fancyhdr, unicode-math, multirow}
\DeclareMathOperator{\Var}{Var}
\DeclareMathOperator{\sd}{SD}
\DeclareMathOperator{\Pois}{Pois}
\DeclareMathOperator{\Bin}{Bin}
\DeclareMathOperator{\NegBin}{NegBin}
\DeclareMathOperator{\sech}{sech}
% Put this in your preamble (you already load array)
\newcolumntype{C}[1]{>{\centering\arraybackslash}m{#1}}
\newcolumntype{L}[1]{>{\raggedright\arraybackslash}m{#1}}

% Text: Calibri, all bold
\setmainfont{Calibri}
\renewcommand{\seriesdefault}{b}

% Math: choose a math font and visually embolden it
\setmathfont{TeX Gyre Termes Math}                  % base math font
\setmathfont[FakeBold=1.9]{TeX Gyre Termes Math}    % embolden; adjust 1.6–2.4


% set header
\pagestyle{fancy}
\fancyhf{} % clear defaults
\lhead{\resizebox{!}{2cm}{H}}
\chead{\raisebox{0.5cm}{\Huge Introduction}}
\rhead{\resizebox{!}{2cm}{H}}
\setlength{\headheight}{2.5cm} % slightly larger than your 2cm image

% start document
\begin{document}
\thispagestyle{fancy} 

\begin{table}[h!]
\centering
\renewcommand{\arraystretch}{1.2} % a little vertical padding
\begin{tabular}{m{1.2in} m{4in}}
% -------------------- Single --------------
\multirow{3}{*}{\centering\fbox{\includegraphics[width=1in]{images/single.png}}} 
  & $A = \{1,2,3,4\}$ \\ 
  & $B = \{3,4,5,6\} $ \\ 
&\\ 
&\\
&\\
% -------------------- Abstract --------------
\multirow{2}{*}{\centering\fbox{\includegraphics[width=1in]{images/abstract.png}}} 
  & $A\cap B = \{3,4\}$ \\ 
  & $A\cup B = \{1,2,3,4,5,6\} $ \\ 
  & $A\cup B \setminus A\cap B = A\cup B - A\cap B = \{1,2,5,6\}$\\
&\\
&\\
% -------------------- Question --------------
\multirow{3}{*}{\centering\fbox{\includegraphics[width=1in]{images/question.png}}} 
& \includegraphics[width=3in]{images/Section_01_P.png}\\
& $ |L \cup D \setminus L \cap D|    = ?$ \\
\end{tabular}
\end{table}

% second page
\newpage
\thispagestyle{fancy}

\begin{table}[h!]
\centering
\renewcommand{\arraystretch}{1.2} % a little vertical padding
\begin{tabular}{m{1.2in} m{4in}}
% -------------------- Answer --------------
\multirow{3}{*}{\fbox{\centering\includegraphics[width=1in]{images/key.png}}} 
& \includegraphics[width=3in]{images/Section_01_P.png}\\
& $ |L \cup D \setminus L \cap D|    = |L| + |D| - 2|L \cap D|$ \\
& $ = 60 + 30 - 2(20) = 50$\\
\end{tabular}
\end{table}
\end{document}
