In a triangle, the interior angles have a sum of 180 degrees. To determine the value of x, you need to first find the measure of the angle supplementary to the exterior angle.
Since the exterior angle is 129 degrees, the other angle supplementary to it would equal 51.
After you know that, add 51 + 55 = 106
180 - 106 = 74, so the measure of x is 74 degrees.