In order to solve this exercise it is importan to remember the definition of Straight angles. A Straight angle is an angle that measures 180 degrees.
By definition, Supplementary angles are those angles that add up to 180 degrees.
Analizing the diagram, you can identify that the angles shown are Supplementary angles. Then, you can set up the following equation:
Now you must solve for "x" in order to find its value. This is:
The answer is: