Hi there!
This is an example of supplementary angles, angles that sum up to 180° (they form a straight line).
Summing these two angles up equals 180°, so:
180° = (10x - 20)° + (6x + 8)°
We can drop the ° sign for ease of solving.
180 = 10x - 20 + 6x + 8
Combine like terms:
180 = (10x + 6x) - (20 + 8)
180 = 16x - 12
Add 12 to both sides:
192 = 16x
Divide both sides by 16:
192/16 = 16x/16
x = 12°