Step-by-step explanation:
the angle CDB is the supplementary angle to 9x+10 (angle ADC).
that means they stand together for 180°. as all angles around a single point on one side of a line are together always 180°.
and then, the sum of all angles in a triangle is always 180°.
so,
CDB + (4x + 10) + 50 = 180
CDB = 180 - (9x + 10) = 180 - 9x - 19 = 170 - 9x
therefore,
(170 - 9x) + (4x + 10) + 50 = 180
170 - 9x + 4x + 10 + 50 = 180
-5x = -50
5x = 50
x = 10
angle CDB = 170 - 9x = 170 - 90 = 80°
