Is this what you mean?. You know, you can make circles with paint.
If so, you can find the radius by using
We find that r=10.
Now that we have r, the rest is easier.
The angle can be found by using the formula for a long chord,
Since r=10, we have , Solve for theta and we get
Then, after you have theta, you can use the formula for the area of a segment of a circle to find its area.
That formula is