\documentclass[a4paper,12pt]{article}
\usepackage{epsfig}
\newcommand{\un}{\underline}
\begin{document}
\parindent=0pt


\begin{center}
\textbf{Vector Fields}

\textit{\textbf{Scalar and Vector Fields}}
\end{center}

\textbf{Question}

Determine the field lines of the following polar vector field.

$\un{F} = \un{\hat{r}} + r\un{\hat{\theta}}$


\textbf{Answer}

The field lines satisfy $dr = d\theta$.

So the field lines are the spirals $r = \theta + C$.

\end{document}