㎡-2mn-3n²=0
(m-3n)(m+n)=0
m=3n或m= -n
当m=3n时
(2m-n)/(3m+n)
=(6n-n)/(9n+n)
=1/2
当m= -n时
=(-2n-n)/(-3n+n)
=3/2