Let $C_1\equiv x^2+y^2-169=0$  and $C_2\equiv x^2+y^2-24x-32y+111=0.$  Let  $AB$  be their common chord. Maximum area of $\Delta ABC$  if $C$  lies on minor arc  $AB$ (Of either of the circle),  is equal to $a\sqrt{30}unit(a\in N).$  Find the value of  $a.$