16) express the complex number in trigonometric form. -3i a) 3(cos 180° + i sin 180°) b) 3(cos 270° + i sin 270°) c) 3(cos 90° + i sin 90°) d) 3(cos 0° + i sin 0°)

now, as far as the angle θ, if we plug those values, we'd get an undefined, it just so happen that tan⁻¹ is not defined on that range, however, let's just use the provided coordinates, check the picture below.

therefore