This problem is very simple, extend the intersection of DO and BC to a point e, and then make the chord center distance of BC after equilateral, let db = x,

OE = x-4, EF = x-5, because there is a 30-degree angle, OE = 2ef, x = 6 to solve. I don't know if I understand, but I can't put it on the picture. This is not up to grade. I am interested in answering questions for the first time. I'm a math teacher, so just ask me what you want!