So this triangle is an iscoseles triangle. In the Isosceles triangle theorem, If two sides of a triangle are equal in measure, then the angles opposite those sides are equal in measure. That means that the other angle is also x. We know that the angles of a triangle add up to 180. So x + x + 80 = 180. 2x + 80 = 180. 2x = 100. x = 50.