Answer
$\left(\frac{3\sqrt2}{2},\frac{3\sqrt2}{2}\right)$
Work Step by Step
By definition: $x=r\cdot \cos(\theta)$ and $y=r\cdot \sin(\theta)$.
Here $r=-3 \text{ and } \theta=\frac{-3\pi}{4}$.
Hence, after plugging these in:
$x=-3\cdot \cos(\frac{-3\pi}{4})=-3\cdot(\frac{-\sqrt2}{2})=\frac{3\sqrt2}{2}$
$y=-3\cdot \sin(\frac{-3\pi}{4})=-3\cdot\frac{-\sqrt2}{2}=\frac{3\sqrt2}{2}$
The point has coordinates $\left(\frac{3\sqrt2}{2},\frac{3\sqrt2}{2}\right)$ in the rectangular coordinate system.