The course of a ship is shown. The ship sails from port A via port B and C to port D. On its way from Port B to port D, The ship turns counterclockwise at port C with an angle b=150 degrees. Ports B, C and D form a circle with center at port D. If AB is tangent to a circle, what is the measure of angle a at port B, at which ship turns left on its way from Port A to port C?
