We have a 30 - 60 -90 right triangle
The side opposite the right angle is 2 /sqrt (3) times the angle opposite the 60° angle
So AC = 2 / sqrt (3) * 15 = 30/sqrt 3 = (30/3)sqrt (3) = 10sqrt 3
And the side oposite the 30° angle = AB = (1/2)*10sqrt (3) = 5 sqrt 3
