We can see that the angles of 39° and 90° in the figure are adjacent, that is, they have one side in common.
We can also see that the angle x includes both smaller angles. The angles x and 39° have one side in common, and the angles x and 90 have one side in common.
Then, we can conclude that the angle x is the sum of those 2 smaller angles in the figure.
So we have that:
So the measure of x is 129°.