\documentclass[12pt]{article}
\RequirePackage{amsmath,amsthm,amssymb,latexsym,amsthm,verbatim,xspace,amstext,xspace,setspace}
\usepackage{parcolumns}
\newcommand{\command}[1]{\colchunk{\begin{description}\item[]{#1}%
    \end{description}}}
\newcommand{\meanin}[1]{\colchunk{\begin{description}\item[]{#1}%
    \end{description}}\colplacechunks}
\setlength{\textheight}{9.2in}
\begin{document}
\begin{center}{ \Large{Mathematica practice}  \\\large{9/12/2017} }\end{center}
\begin{parcolumns}[colwidths={1=2 in},nofirstindent]{2}
\command{$a=\{1,2,3\}$}\meanin{assign a vector}
\command{$a.b$}\meanin{take dot product of two vectors}
\command{Cross[$a,b$]}\meanin{take cross product of two vectors}
\command{Norm[$a$]}\meanin{compute the length of a vector}
\end{parcolumns}
\vspace{0.5in}\textbf{Practice}\\
$a=\{1,2,3\}$, $b=\{1,-1,0\}$, $c=\{0,1,2\}$.
Compute $\|a+2b-c\|$, $(a\times b)\times c$ and $a\cdot(b\times c)$.\\
\hrule
\begin{parcolumns}[colwidths={1=2 in},nofirstindent]{2}
\command{$A$=\{\{1,2,3\},\{4,5,6\},\{7,8,9\}\}}\meanin{assign a matrix}
\command{MatrixForm[$A$]}\meanin{show matrix in planar form}
\command{Det[$A$]}\meanin{compute the determinant of a matrix}
\command{A[[i,j]]}\meanin{the entry at the $i$'th row and $i$'th column}
\command{A[[i]]}\meanin{the $i$'th row}
\command{A[[All,j]]}\meanin{the $j$'th column}
\end{parcolumns}
\vspace{0.5in}\textbf{Practice}\\ Execute the above commands.\\
\hrule
\vspace{0.2in}\noindent\textbf{Practice}\\
Write the parametrization of a line passing through point $A(1,0,-1)$ with direction vector $a=\langle 2,1,1\rangle$.
\newpage
\begin{center}\Large Some plotting commands\end{center}
\begin{parcolumns}[colwidths={1=2 in},nofirstindent]{2}
\command{Plot}\meanin{plot function $y=f(x)$}
\command{Plot3D}\meanin{}
\command{ContourPlot}\meanin{}
\command{ContourPlot3D}\meanin{}
\command{ParametricPlot}\meanin{}
\command{ParametricPlot3D}\meanin{}
\end{parcolumns}
\vspace{1in}
\begin{center}\Large Some options\end{center}
\begin{parcolumns}[colwidths={1=2 in},nofirstindent]{2}
\command{Axes$\to$True/False}\meanin{Show/Hide axes}
\command{AxesLabel$\to$\{\ldots\}}\meanin{}
\command{Mesh$\to$True/False}\meanin{Show/Hide mesh}
\command{ViewPoint$\to$\{\ldots\}}\meanin{}
\end{parcolumns}
\end{document}