Measure of all angles must equal 180 degrees.
Therefore,
(4x+20) + (2x+34) + x = 180
6x + 54 + x = 180
7x + 54 = 180
7x = 126
x = 18
4(18)+20 = 92
Angle A = 92
2(18)+34 = 60
Angle C = 60 degrees
Unknown angle is 18 degrees because the "x" represents the third angle.