The triangle angle sum theorem states that the 3 interior angles of a triangle sum to 180 degrees. Add all of the angle measures together and set equal to 180. (3x-4)+(5x-7)+(13x+2) = 180. Combine like terms. 21x -9 = 180. Add 9 to both sides. 21x= 189. Divide both sides by 21. X = 9
