Multiply : \( (x^2 + 4y^2 + z^2 + 2xy + xz – 2yz) \) by \( (x – 2y – z) \)
\[
(x^2 + 4y^2 + z^2 + 2xy + xz – 2yz)(x – 2y – z)
\]
\[
= x^3 – 2x^2y – x^2z
+ 4x y^2 – 8y^3 – 4y^2z
+ xz^2 – 2yz^2 – z^3
\]
\[
\quad + 2x^2y – 4xy^2 – 2xyz
+ x^2z – 2xyz – xz^2
– 2xyz + 4y^2z + 2yz^2
\]
\[
= x^3 – 8y^3 – z^3 – 6xyz
\]