a^(m-n)=a^(6m+4n-5m-5n)=a^(2(3m+2n)-5(m+n))
=a^2(3m+2n)/a^5(m+n)
=72^2/6^5
=(2*6^2)^2/6^5
=4*6^4/6^5
=4/6
=2/3