[OS X TeX] Indenting problem

[David Arnold]

All,

I hae the following code:

\documentclass{memoir}
\usepackage{amsmath}
\usepackage{tkz-euclide,tkz-fct}
\usepackage{hyperref}
\usepackage{xcolor}
\usepackage{multicol}
\definecolor{shadecolor}{gray}{0.9}
\definecolor{darkred}{rgb}{0.626,0,0}
\newcounter{exercise}[section]
\renewcommand{\theexercise}{\arabic{exercise}}
\newdimen\probindent  \probindent=20pt
\newcommand\itembox[1]{\hbox to \probindent{\hfil #1}}
\newcommand\newindent{\hskip 1.2\probindent}
\newenvironment{exercise}%
	{\refstepcounter{exercise}
	\par\hangindent 1.2\probindent\noindent\itembox{\textbf{\theexercise}.}}
	{\par\medbreak}

\begin{document}

% Exercise #21
\begin{exercise}\label{ex:3-4-21}
Assume that the relationship between an object's velocity and its time is linear. At $t=0$ seconds, the object's initial velocity is $20$ m/s. It then begins to speed up at a constant rate of $5$ meters per second per second. 
\begin{enumerate}[a) ]
\item Set up a coordinate system, placing the time $t$ on the horizontal axis and the velocity $v$ on the vertical axis. Label and scale each axis. Include units in your labels.
\item Use the initial velocity and the rate at which the object's velocity is increasing to draw the line representing the object's velocity at time $t$. Use the slope-intercept form to determine the equation of the line.
\item Replace $x$ and $y$ in the equation found in part~(b) with $t$ and $v$, respectively. Use the result to determine the velocity of the object after 14 seconds.
\end{enumerate}


\end{exercise}

\end{document}

The problem is that the item labels stick out in the margin instead of being aligned with opening paragraph.

We tried using minipage, but that got me into trouble with our two column exercise sets.

Workaround?

D.