2b=a+c, then (2b) 2 = (a+c) 2 = A2+C2+2ac = A2+C2+12, that is, 4b 2 = a 2+c 2+65438.

b^2=a^2+c^2-2accosb=a^2+c^2-√3ac=a^2+c^2-6√3……(2)

If (1)-(2): 3b 2 = (12+6 √ 3), then b 2 = 4+2 √ 3, so b=√3+ 1.