Mechanics of machine force analysis with analytical method

[Pipsqueakalchemist]

So I was looking at this example problem in my textbook and I don’t understand how they got -168.1 degrees. The part I’m confused with is the first part of the 2nd picture, there’s a coma in the inverse tan, I don’t know what that means.

 

[Baluncore]

To resolve the full circle of slope or vector direction, the arctangent function needs both the dy and the dx components as parameters.
https://en.wikipedia.org/wiki/Atan2