Pool ball hits table, find an angle

[mbrmbrg]

Figure 9-47 (attatched) shows a 0.165 kg cue ball as it bounces from the rail of a pool table. The ball's initial speed is 1.60 m/s, and the angle 1 = 30.0°. The bounce reverses the y component of the ball's velocity but does not alter the x component.

(a) What is theta_2?
(b) What is the change in the ball's linear momentum in unit-vector notation?

I got the physics of part b, the trig of part a is messing me up. I went so far as to make a system:
$$v\cos\theta_2=-v\cos\theta_1$$
$$v\sin\theta_2=v\sin\theta_1$$

As I thought! The angle must be in the second quadrant... so that means add 90 degrees, right? (And not 180-angle?)
But I don't know how to go from the typical coordinate system used for measuring angles to the funky one shown in the diagram.

 

[mbrmbrg]

Never mind. I got the correct answer of 30 degrees (I had started out by saying theta_2 = -30 degrees). But why? How is that allowed? What kind of crazy notation are they using here (that apparently is standard...)?