28 degrees S is 180-28=152 degrees N. Angular distance between A and B is 152-30=122 degrees. The length of the sector divided by the circumference of the earth is 122/360, so s/2(pi)r=122/360, where r=6400 and s is AB.
s=2(pi)*6400*122/360=39040(pi)/9=13,627.5 km approx.