Itemize using multicol package

Here's a solution that doesn't use a multicols environment. Instead, it loads the enumitem package with the option inline and uses an enumerate* environment. The solution also employs the machinery of the amsthm package to streamline the definition of the problem environment.

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%% (I've simplified the preamble code to the bare minimum)
\theoremstyle{definition} % upright-lettering style

\usepackage[inline]{enumitem} % <-- note the option 'inline'


\setcounter{problem}{6} % adjust as needed
Calculate the following determinants.

\item $\begin{vmatrix} 3 & 0 \\ 2 & 6 \end{vmatrix}$ 
\hspace*{1cm} %  choose a suitable amount of horizontal whitespace
\item $\begin{vmatrix} a & a \\ b & b \end{vmatrix}$ 
\item $\begin{vmatrix} a+b & a-b \\ a-b & a+b \end{vmatrix}$ 
\item $\begin{vmatrix} 3^t & 2^t \\ 3^{t-1} & 2^{t-1} \end{vmatrix}$


Another fast solution could be:

  1. to use \usepackage[margin=1in]{geometry};
  2. enumitem inline \usepackage[inline]{enumitem};
  3. To write correctly the matrices \begin{vmatrix}....\end{vmatrix};

\item[\hskip \labelsep {\bfseries #1}\hskip \labelsep {\bfseries #2.}]}{\end{trivlist}}
\usepackage{multicol} % to insert columns
\usepackage{parskip} % For no indentation and a bit of space in paragraphs

% to format enumerate to letters in line

\title{Problem Set 2}
\author{Ruben Perez Sanz}
\date{7 September, 2020}

    Calculate the following determinants:
            \item $\begin{vmatrix} 3 & 0 \\  2 & 6 \end{vmatrix}$
            \item $\begin{vmatrix} a & a \\  b & b \end{vmatrix}$
            \item $\begin{vmatrix} a+b & a-b \\  a-b & a+b \end{vmatrix}$
            \item $\begin{vmatrix} 3^t & 2^t \\  3^{t-1} & 2^{t-1} \end{vmatrix}$


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