Given: tangent to circle o.

if m = 150á, then a =

75
105
150

The degree measure of an arc is equal to the measure of central angle that intercept the arc.
m ( arc DB ) = 150°
m ∠ B O D = 150°
∠ A is inscribed angle, which is supplement of the 1/2 of the central angle ( A is in the minor arc ):
m∠ A = 180° - (150°/2 ) = 180° - 75° = 105°

B ) 105°