Find the product:

$(x-y-z)\left(x^{2}+y^{2}+z^{2}+x y-y z+x z\right)$

$=(x+(-y)+(-z))\left(x^{2}+y^{2}+z^{2}+x y-y z+x z\right)$

We know

$(a+b+c)\left(a^{2}+b^{2}+c^{2}-a b-b c-c a\right)=a^{3}+b^{3}+c^{3}-3 a b c$

Here, $a=x, b=-y, c=-z$

$(x+(-y)+(-z))\left(x^{2}+y^{2}+z^{2}+x y-y z+x z\right)=x^{3}-y^{3}-z^{3}-3 x y z$