x = 90

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 same 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.