A straight line is 180°. So you could do:
(6x + 12) + (16x + 4) + (18x + 4) = 180°
You add all of the individual angles and set it equal to 180 because they should equal a total of 180°.
6x + 12 + 16x + 4 + 18x + 4 = 180
Combine like terms
40x + 20 = 180
Subtract 20 on both sides
40x = 160
Divide 40 on both sides to get "x" by itself
x = 4
