I don't understand your question - are you using to denote the angle made with the axis? Or are you using it to denote an coordinate?
If you are referring to an angle, you should use .
The "slope" of a line is the tangent of the angle, , so the angle is the inverse tangent:
I don't understand the "vice-versa" part of your question. Your original question was "how do I find the angle" so "vice-versa" would be "how does the angle find me"!