When calculating angles using a circle theorem ... so with the chord they form an isosceles triangle. Angles in a triangle add up to 180°.

Angle OGK (\(x\)) = angle OKG because triangle GOK is also isosceles. Lengths OK and OG are also both radii. Angle GOH = \(180 - 2y\) (because angles in a triangle add up to 180°) Angle GOK ...