3^m=6
27^n=2,(3^3)^n=2,3^3n=2
(3^n)^3=2
3^n=三次根号2
3^(m+n)=3^m*3^n=6*三次根号2
3^(2m-3n)=3^2m/3^3n=(3^m)^2/2=6^2/2=18