Thanks!
y = -√3 x
y = r sin θ
x = r cos θ
So we have that
r sin θ = -√3 (r cos θ ) divide both sides by r cos θ
sin θ / cos θ = -√3
tan θ = - √3 take the tangent inverse
arctan ( -√3) = θ = - pi/3
Add pi to this
- pi/ 3 + pi = θ
(2/3) pi = θ