We will use the next image
the angles in red are equals
The angle in blue is 30°
We need to remember that the sum of the interior angles of a triangle must be 180°
x is the red angle
2x+30=180
2x=180-30
x=150/2
x=75
the angle DBC can be calculated as a complementary angle, the complementary angle is formed with two angles that form an angle of 90°
angle DBC =90-75
angle DBC=15° the angle in green in the image
