The sum of the 3 outside angles of a triangle equal 360 degrees.
Add the equations together to equal 360:
x+8 + x +20 + 2x +12 = 360
Combine the like terms on the left side of the equal sign:
4x + 40 = 360
Subtract 40 from both sides:
4x = 320
Divide both sides by 4:
x = 80