The 2 angles are supplementary angles and need to equal 180 degrees.
The sum of both angles equal 180:
8X + 12X = 180
Add:
20X = 180
Divide both sides by 20:
X = 180/20
X = 9
Now replace X with 9 and solve:
8X = 8(9) = 72 degrees
12X = 12(9) = 108 degrees