There two things you need to remember with regards to your figure. Firstly, the sum of all angles in a triangle is equal to 180.
Secondly, supplementary angles are two angles that form a straight line. In other words, the two lines sum up to 180 degrees.
Looking at your figure and the rules written above, you can then assume that the measure of angle x is equal to the sum of the two angles given inside the triangle.
