The complex number -6i equals 0-6i and is equal to the vector 
whose tail is at the origin and whose tip is at the point (0,-6).
It is 6 units long, so its mudulus r is 6 and its argument q is 270°,
indicated by the counter clockwise arc from the right side of the
x-axis to the vector which represents the complex number:



So the complex number = -6i = 6(cos270° + i∙sin270°) 

Edwin