Angles x and z are supplementary, meaning that they add up to 180. From the markings on the diagram, angle z and the angle measuring 20 are complementary, meaning they add up to 90. So the game plan is to find out the measure of angle z first, then use that to find the measure of angle x.
angle z = 90 - 20 so
angle z = 70 degrees. And
angle z + angle x = 180. Subbing in our value for z:
