Finding the angle of $-3+5i$ To find the angle $\theta$ of this complex number I know that I have to imagine it in the complex graph, draw a triangle and then calculate the arctan.
Here is the representation of $-3+5i$ on the graph:

(I'll explain $\alpha$ and $\phi$ in a moment)
I know that since I have the adjacent and opposite sides of the triangle I can correlate them with the angle using the tangent. Then, I need to calculate the arctan to get the angle.
But my problem is finding the tangent, because I don't know which is the right triangle in this case.
My question is: 
 is $tan \theta = \frac{5}{-3}$ or $tan \theta = \frac{-3}{5}$?
How do I know whether if $\theta$ is supposed to be $\alpha$ or $\phi$?

By convention your $\theta$ is defined to be 

the angle made with the positive $X$-axis in anticlockwise direction

In this case it will be $90^\circ+\alpha$
