5^2*3^(2n+1)*2^n - 3^n*6^(n+2) = 25* 3^(2n+1)*2^n - 3^n*3^(n+2)*2^(n+2)
=25*3^(2n+1)*2^n - 3^(2n+1)*3*2^n*4
=3^(2n+1)*2^n[25-12]
=13*3^(2n+1)*2^n
是13的倍数.