the answer is 43 degrees.
in order to find x, you need to know the two angles on the base of the triangle.
by using vertical angles theorem, you find that the bottom right angle is 80 degrees.
by noticing the missing angle is vertical to the supplementary angle of 123, you can subtract 123 degrees from 180 and get 57.
then, because every triangle has a sum of 180 degrees, you add 57 degrees and 80 degrees together to get 137 degrees.
finally, to get x, you subtract 137 degrees from 180 and get x=43 degrees.
i hope that i helped!