△=9m²-6mn+n²+8mn
=9m²+2mn+n²
=8m²+(m²+2mn+n²)
=8m²+(m+n)²
m>0则9m²>0
所以△>0
所以有两个不相等的实数根
x1+x2=(3m-n)/2
x1x2=-mn/2
(x1-n)(x2-n)
=x1x2-n(x1+x2)+n²
=-mn/2-n(3m-n)/2+n²
==(-4mn+3n²)/2
=n(3n-4m)/2
m>n>0
所以3n-4m