Answer
$\color{blue}{{\bf a.} -3 + 3i\sqrt{3}}$
Work Step by Step
$\begin{align*}
6\, \text{cis}\, \frac{2\pi}{3} &= 6\left(\cos\frac{2\pi}{3} + i\sin\frac{2\pi}{3} \right) \\
&= 6\left(-\frac{1}{2} + \frac{\sqrt{3}}{2}i\right) \\
&= -\frac{6}{2} + \frac{6\sqrt{3}}{2}i \\
&= -3 + (3\sqrt{3})i \\
\color{blue}{6\, \text{cis}\, \frac{2\pi}{3}}\ &\color{blue}{= -3 + 3i\sqrt{3} \qquad\text{Choice}\ {\bf a.}}
\end{align*}$
