Answer
$(-6,-2\sqrt3)$
Work Step by Step
To convert from polar to rectangular coordinates we use $x=r\cos\theta$ and $y=r\sin\theta$. To convert from rectangular to polar coordinates, we use $r=\pm\sqrt{x^2+y^2}$, $\tan\theta=\frac{x}{y}$, but aware of which quadrant the point lies in.
Hence here the given point in rectangular coordinates: $(-4\sqrt3\cos(\pi/6),-4\sqrt3\sin(\pi/6))=(-6,-2\sqrt3)$