\documentclass[12pt]{article}

\title{Math 451, Homework 1}
\author{Your name}
\date{}



\begin{document}
\maketitle


\section*{3.12}
Type your solution here.

\paragraph{Case 1: $a \ge 0$.}
Here we proceed as follows \ldots

\paragraph{Case 1: $a < 0$.}
Here the argument uses Theorem 1.14 from the book.


\section*{5.17}
Type your solution here. And so on.


\end{document}