check the picture below.
by the "inscribed angle theorem", we know the arc at "x" is 116°, a full circle will have 360°, therefore if we go around the O vertex, the interior angle at O is 360° - x, or 244°.
well, the sum of all interior angles in a polygon is
180(n - 2), where n = number of sides.
in this case, we have a "concave quadrilateral" inside, so it has 4 sides, so the sum of all its interior angles is 180( 4 - 2), or 360°.
we know the other angles, thus y = 360 - 244 - 38 - 58
