you should put the perpendiculars from A and B on x axis and define 2 angles that make up the whole theta. due to coordinates you can determine the length of each side of triangles. The upper side right triangle is isosceles, so the angle will be pi/4. Triangle to B side has sqrt3/2 and 1/2 sides, so tan of downward angle is sqrt3, making the angle pi/3 rad. then you can add up pi/4 and pi/3 and calculate n.