B is BD⊥AC, AC is D.

∵BD⊥AC

∴ Deriving RT△ABD

∠∠ADB+∠A+∠ABD = 180，∠A=60，∠ADB=90

∴∠ABD=30

∫RT△ABD

∴ ad =1* ab (the right angle side of RT triangle facing 30 degrees is half of the hypotenuse)

∫AB = 15

∴AD=7.5

Derived from Pythagorean theorem

BD=( 15 root number 3)/2

∫AC = AD = DC，AD=7.5

∴DC=33/2

∵BD⊥AC

∴RT△BDC

Derived from Pythagorean theorem

BC^2=DC^2+BD^2

BC=2 1cm

Hope to adopt