I'm really confused on which angle here is "the angle at Ox", and how you find out.

That's a quick image I made of it. I have that as the vector $\displaystyle |-3i + 2j|$ but I'm not sure whether the angle is $\displaystyle a$ or $\displaystyle a + b$ or $\displaystyle b + c$ or $\displaystyle a + b + c$.