Answer
$-3i$
Work Step by Step
We know from the unit circle that:
$\cos \dfrac{3 \pi}{2}=0$ and $\sin \dfrac{3 \pi}{2}=-1$
Thus,, we simplify the given expression as follows:
$3\left[\cos (\dfrac{3\pi}{2})+i \ \sin (\dfrac{3 \pi}{2})\right]
\\=3 (0-i)
\\=3(0)-3(i)
\\= -3 i$
