More simply, the angle at the centre is double the angle at the circumference. Angle OGK (\(x\)) = angle OKG because triangle GOK is also isosceles. Lengths OK and OG are also both radii. Angle GOH = ...
It is important to remember other angles rules, such as angles on a straight line add up to 180°, and to recognise angles in parallel lines. Make sure you are confident in applying Pythagoras’ theorem ...