So you are trying to find the angle of x. The sum of all the angles in a triangle is 180. Now, using supplemental angles shows that a straight line is 180° (a semi circle) So when they cut out a portion of the angle with a line it becomes two portions that add up to 180. 180-110 is 70°. If you continue the horizontal line, because the triangle lines were congruent the angles are the same. Now you have 70+ 70 + x = 180, which means x = 40°