Math

Angle DAB  =90

So angle DAE  = 90 / 3 =  30

And DAE  is a 30- 60 -90 right triangle

DE is  opposite the 30 degree angle

And AD is opposite the 60 degree angle

So AD =  6sqrt 3  =  BC

So

BF  = BC - FC   =   6sqrt 3  - 1  =   5sqrt 3