Hey there!
What are supplementary angles?
The angles whose sum make up 180 degrees are called supplementary angles.
Angle A + Angle B = 180
... 9x + 20 + 6x + 10 = 180
... 15x + 30 = 180
... 15x = 150
... x = 150 / 15
... x = 10
We know that the angle B is 6x + 10
... 6x + 10
... 6 ( 10 ) + 10
... 60 + 10
... 70
Hence, the angle B is 70 degrees.
Hope it helps!