Been awhile for geometery, but m<DAB = 90Âº. A is on the circle ... arc BD is 180Âº since BD is the diameter. Angle is Â½ the arc.
m<DBA = 30Âº ... Â½ the arc length of arc DA.
m<BDA = 60Âº ... triangle adds to 180Âº in addition Â½ the arc length remaining.
Since a Rt Triangle... trig works (or faster if you know the 30-60-90 triangle relationships!
sin <ABD = L/18
L = 18 (sin 30)
L = 18(Â½)
L = 9 cm