(x+y+3)(x-my+n)=x平方-xy-2y平方+5x-4y+6
x²-mxy+3x+xy-my²-3my+nx+ny+3n=x²-xy-2y²+5x-4y+6
x²-(m-1)xy-my²+(3+n)x+(n-3m)y+3n=x²-xy-2y²+5x-4y+6
∴m-1=1
-m=-2
3+n=5
n-3m=-4
3n=6
∴m=2
n=2