Could you please explain what are the points A,B,C,O,X?

As for the first question, the arc length S of a circular sector, is connected to the radius r and the epicentric angle a (measured in radians), by the formula S=ra.

The length l of the chord (BC, if I am guessing the points right)

is l=2rsin(a/2).

And for the area E of the circlular segment, we have E=(1/2)ar^2.

These are enough to calculate all else