Missing $ inserted while using Cases environment

Question

f(n) = \begin{cases} n/2 &\mbox{if } n \equiv 0 \\
(3n +1)/2 & \mbox{if } n \equiv 1 \end{cases}

I read somewhere that in the cases environment it doesn't require $ and when I do try and put $, it says I am still missing it. I am fairly new to Latex and quite confused.

\documentclass[12pt, letterpaper]{article} 
\usepackage[utf8]{inputenc}
\usepackage{graphicx}
\usepackage{amsmath}
\usepackage{amssymb}
\setlength{\parindent}{0pt}

\title{}
\author{} 
\date{} 

\begin{document} 
\maketitle
\begin{tabbing}

\underline{\textbf{Problem 13.}} \\

f(n) = \begin{cases} n/2 &\mbox{if } n \equiv 0 \\
(3n +1)/2 & \mbox{if } n \equiv 1 \end{cases}

\end{tabbing}
\end{document}

Answer 1

In relation by comments of the very good users I add a MWE. I have delete \setlength{\parindent}{0pt} and I have added the package \usepackage{parskip} with the same function, \text{if } instead of \mbox{if }.

\documentclass[12pt,letterpaper]{article} 
\usepackage[utf8]{inputenc}
\usepackage{graphicx}
\usepackage{amsmath}
\usepackage{amssymb}
\usepackage{parskip}
\title{}
\author{} 
\date{\today} 

\begin{document} 
\maketitle
\begin{tabbing}
\underline{\textbf{Problem 13.}} \\
$f(n) = \begin{cases} n/2 &\text{if } n \equiv 0 \\
(3n +1)/2 & \text{if } n \equiv 1 \end{cases}$
\end{tabbing}
\end{document}