Please show work and explanation

Writing a complex number in trigonometric form: Special angles

Write the complex number z=-2i in trigonometric form( sometimes called polar form)

The r is equal to square root of 0 squared plus -2 squared which equals 2, but when I'm trying to find theta which is b/a or-2/0 it is undefined. How can I find the value of theta and write it in z=r(cos(theta) plus isin(theta))