Why would $(2,3\pi)$ and $(2,-3\pi)$ be different points in polar coordinates?

Question

I am looking at this problem solution and it says $(2,3\pi)$ and $(2,-3\pi)$ represent two different points in polar coordinates. Why would they be different? In my opinion, since I am adding $6\pi$ (which is a multiple of $2\pi$) to $-3\pi$, they should be the same. Even plotting the two points gave me an indication that they are the same. Where am I mistaken?

EDIT: Here is a similar problem part (b)