27分之一*a^(3m-n)-3分之一*a^(m-n)*b^2n
=1/3[1/9a^(m-n)a^2m-a^(m-n)b^2n]
=1/3a^(m-n)[(1/3a^m)^2-(b^n)^2]
=1/3a^(m-n)(1/3a^m+b^n)(1/3a^m-b^n)