You can use a minipage
as follows:

\documentclass{article}
\begin{document}
\begin{itemize}
\item Riemann Sum
\item Newton Cotes formulae of different degrees
\(
\left\{ \quad
\begin{minipage}[c]{0.3\linewidth}
\item Trapezoidal Rule
\item Simpson's \( 1/3 \) Rule
\item Simpson's \( 3/8 \) Rule
\end{minipage}
\right.
\)
\end{itemize}
\end{document}
Alternatively you can use a tabular
:
\documentclass{article}
\begin{document}
\begin{itemize}
\item Riemann Sum
\item Newton Cotes formulae of different degrees
\(
\left\{
\begin{tabular}{@{\textbullet\enspace}l}
Trapezoidal Rule \\
Simpson's \( 1/3 \) Rule \\
Simpson's \( 3/8 \) Rule
\end{tabular}
\right.
\)
\end{itemize}
\end{document}

This avoids having to specify the width, but means that you have to provide the itemize
-like markup yourself.
As Schweinebacke points out there is also the varwidth
environment from the varwidth
package, that will replace minipage
and you just specify a maximum width. However it behaves slightly differently: the itemize list is now at second level, so by default bullets are replaced by dashes, and there is a considerable left margin indentation. So if want the same type of appearance as before you need to modify the itemize
style, e.g. via the enumitem
package:
\documentclass{article}
\usepackage{varwidth}
\usepackage{enumitem}
\begin{document}
\begin{itemize}
\item Riemann Sum
\item Newton Cotes formulae of different degrees
\(
\left\{
\begin{varwidth}{\textwidth}
\begin{itemize}[label=\textbullet,leftmargin=1em]
\item Trapezoidal Rule
\item Simpson's \( 1/3 \) Rule
\item Simpson's \( 3/8 \) Rule
\end{itemize}
\end{varwidth}
\right.
\)
\end{itemize}
\end{document}
If you really want to avoid math mode and use tikz
then you can use
\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{decorations.pathreplacing,calc}
\begin{document}
\begin{itemize}
\item Riemann Sum
\item Newton Cotes formulae of different degrees
\begin{tikzpicture}[baseline]
\node[inner ysep=0pt] (A) at (0,0) {
\begin{tabular}{@{\textbullet\enspace}l}
Trapezoidal Rule \\
Simpson's \( 1/3 \) Rule \\
Simpson's \( 3/8 \) Rule
\end{tabular}};
\draw [decoration={brace,amplitude=0.5em},decorate,thick]
(A.south west) -- (A.north west);
\end{tikzpicture}
\end{itemize}
\end{document}
which is at tabular at heart. In my opinion, the math mode option is easier.
tikz
avoid math mode, buttikz
is a big machine too.