The two angles marked y ^° are equal because they are in an isosceles triangle. For the same reason, the angles z^°are equal. Since an exterior angle of a triangle is the sum of the two interior and opposite angles, it follows that a=2y and b=2z. Now a^°+ b^° = 180^° since they are the base angles of a parallelogram. So 2y + 2z = 180 giving y+z=90. But, from the angle sum of a triangle x+y+z=180; hence x = 90.