I am unsure of my solution to this question, since the definition of the complex logarithm is somewhat complex.
Since $-3i = 3e^{i\frac{3}{2}\pi}$ we get that $e^z=3e^{i\frac{3}{2}\pi}$
So if we use logarithm function like we did in $\mathbb R$ we would get
$z=\ln(3e^{i\frac{3}{2}\pi})=\ln3+\ln(e^{i\frac{3}{2}\pi})=\ln3+i\frac{3}{2}\pi$
Is this the correct answer? Are there more correct answers?
An idea: putting $\;z=x+iy\;,\;\;x,y\in\Bbb R\;$
$$-3i=e^{z}=e^x\cos y+ie^x\sin y$$
Now compare real and imaginary parts and stuff.