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 .