Hi, there-- Thank you for your clarifying question. When you extend a side in a triangle, then a linear pair of angles automatically appears. Linear pairs are angles that are adjacent and supplementary. Adjacent means that they share a common side. Supplementary means that the sum of their measures is 180 degrees. One of the angles in the linear pair is an interior angle in the triangle, and its supplement is called an exterior angle of the triangle. Here is a link to a pretty nice picture of what I'm talking about: http://hotmath.com/hotmath_help/topics/exterior-angle-theorem.html In vksarvepalli's solution, he assigned the variable x to be the measure of the interior angle. Since the interior angle and the exterior angle form a linear pair, they are supplementary. This means that the measure of the exterior angle will be 180-x because x + (180-x) = 180. Hope this helps! Feel free to email me if you still have questions about this. Ms.Figgy math.in.the.vortex@gmail.com