The Isosceles Triangle Theorem, All interior angles of a triangle add up to 180 degrees. Basically the equation would be (angle 1) + (angle 2) + (angle 3) = 180. whichever angle you dont have you replace with x and solve for it. For example if you had a triangle with two congruent angles of 36 then it would be 36+36+x=180 and you would solve it like any other equation eventually showing you that x = 108. it also works backwards if you are trying to solve for the 2 missing congruent angles.
