How to arrive from $x^2y^2－x^2－y^2－6xy＋4$ to $(xy＋x＋y－2）(xy－x－y－2）$? How do we arrive from 

$x^2y^2－x^2－y^2－6xy＋4$

to 

$(xy＋x＋y－2）(xy－x－y－2）$

?
According to my book these two are equal, but I can't understand how to transform one to another.

$$x^2y^2-x^2-y^2\color{red}{-6xy}+4=x^2y^2\color{red}{-4xy}+4-x^2\color{red}{-2xy}-y^2\\=(xy-2)^2-(x+y)^2=(xy+x+y-2)(xy-x-y-2)$$
