Let, z = -3i

Let 0 = rcosθ and -3 = rsinθ

By squaring and adding, we get

(0)² + (-3)² = (rcosθ)² + (rsinθ)²

⇒ 0+9 = r²(cos²θ + sin²θ)

⇒9 = r²

⇒ r = 3

∴ cosθ= 0 and sinθ=-1

Since, θ lies in fourth quadrant, we have

Thus, the required polar form is