\documentclass[12pt]{article}
\usepackage[margin=1.25in]{geometry}
\usepackage{fancyhdr}
\usepackage{setspace}
\pagestyle{fancy}
\usepackage{amsmath}
\usepackage{amsfonts}
\title{Homework}
\lhead{Class - Section}
\chead{Problem Set X}
\rhead{Your Name}
\begin{document}
\doublespacing
\begin{enumerate}
\item[1.] question 1
\begin{enumerate}
\item[a)] part a
\item[b)] part b
\end{enumerate}
\item[2.] question 2
\end{enumerate}
\end{document}
