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