x^2+4xy+4y^2+x+2y-6=(x+2y)²+(x+2y)-6=[(x+2y)+3][(x+2y)-2]=0
得(x+2y)=-3或2
(x+2y)=-3时6/(x+2y)=-2
(x+2y)=2时6/(x+2y)=3