You must draw a diagram. Inscribe a triangle in a circle.
Draw three radial segments, one from each vertex to the center,
Now you have three sub-triangles. The sum of their areas is the area you want.
The radius of the circle is . How and why?
The angle subtending the arc of length 5 measures . Again how and why?.
If are the length of two sides of a triangle and is the measure of the angle between then the area to that triangle is .