You can only plot on a complex plane, if you look at the article, it also shows you how you go about finding to polar form (which involves trig).

You should be able to work it from there, but if you need help, here is the worked solution for converting it to polar form:

Find the argument (aka the magnitude or the size of the line):

Find the angle with the horizontal:

We know the point is in the second quadrant, therefore:

This finally leaves us with: