\documentclass[11pt]{article}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amsthm}
\usepackage{mathrsfs}
\usepackage{graphicx}

\textwidth=6.5truein \textheight=9truein \oddsidemargin=-0.0truein
\newcommand{\R}{{\mathbb R}}
\newcommand{\Z}{{\mathbb Z}}
\newcommand{\Q}{{\mathbb Q}}
\newcommand{\C}{{\mathbb C}}
\newcommand{\N}{{\mathbb N}}
\newcommand{\mtx}[2]{\left(\begin{array}{#1} #2 \end{array}\right)}
\DeclareMathOperator{\lcm}{lcm}

\begin{document}
\pagestyle{empty}

\voffset -.8 true in %\hoffset -.1 true in
\Large
\noindent

In this .tex file I have started to use a new header, one that
uses the commands from the {\tt amsmath} package, which is built 
into MikTeX and probably the other versions of \LaTeX\ that you are 
likely to find.

It is possible to create your own commands, maybe for a compound
symbol or to avoid retyping something repeatedly, by making your own
``definitions'' (the {\tt amsmath} package calls it a 
``newcommand''), putting 
into the file (before the ``begin document'') so that if you use the 
command word you created for them, the whole definition
will essentially be placed in the file at that point.  I use the 
the symbols for the integers, the real numbers, and so on, in
``blackboard bold'' type face, so I use definitions to simplify 
the commands to get them: 
$$
\Z\qquad \R \qquad \N \qquad \Q \qquad \C 
$$

Notice the one for matrix: It takes two arguments; the first is where 
to put entries in each column --- left, center or right --- and the 
second is the entries, with row entries
separated by ampersands and rows separated by double-backslashes.
\newline
$$
\mtx{ccr}{x&x+1&x+2\\x+3&x^2&2x}
$$

\end{document}