Question

Express the complex number in trigonometric form.

-3i

Answer 1

-3i
is the same as
0-3i
which is the same as
3(0-1i)

so can you find a theta satisfying both of the equations:
\[\cos(\theta)=0 \\ \sin(\theta)=-1\]

Answer 2

you may use that(unit circle) to find the \(\theta\) corresponding to \((\cos,\sin)= (0,-1)\)