x³y^(m-1)·x^(m+n)y^(2n+2) = x^9y^9x^(m+n+3) y^(m+2n+1) = x^9y^9所以：m+n+3 = 9 m+2n+1=9所以n=2,m=4所以：3m-4n = 12-8 =4
