Geometry Problem

Since  AD  is a diameter, then ABD will form a right triangle with angle ABD  = 90°

So...angle ADB   = 40°

And we can find AB  as :

sin (40)  = AB  / 36

AB  = 36 * sin (40)  = 23.140353948732  ≈   7.3658 pi  units

Here is another method:
If we designate O as the center of the circle, then:
Line AO = BO =radius of the circle = 18
ABO is an isosceles triangle with angles =50, 50, 80
18/sin(50) =AB/sin(80), and:
AB =23.14035 =7.3658pi

It sure is 7.3658 pi units!