Factor $3x^2-11xy+6y^2-xz-4yz-2z^2$ This problem is from my Math Challenge II Algebra class, and it's really confusing. How can you factor something like this? Here's the question again: Factor $3x^2-11xy+6y^2-xz-4yz-2z^2$.
 A: Basically by educated guesses. The factorization, if it exists, looks like $(ax+by+cz)(dx+ey+fz)$. Concentrate first on the $x^2$, $xy$, and $y^2$ terms and try to factor that. That should give you $a$, $b$, $c$, and $d$. Then look at the $z$ terms and figure out what $c$ and $f$ are.
A: First, split up the expression into pieces.
\begin{align*}
F(x,y,z) &= 3x^2-11xy+6y^2-xz-4yz-2z^2 \\
&= 3x^2\underbrace{-11xy-xz}_{G(x,y,z)}+\underbrace{6y^2-4yz-2z^2}_{H(y,z)}
\end{align*}
Start with the easy one : $H(y,z)$.
\begin{align*}
H(y,z) &= 6y^2-4yz-2z^2 \\
&= 6y^2-6yz+2yz-2z^2 \\
&= 6y(y-z)+2z(y-z) \\
&= 2(y-z)(3y+z)
\end{align*}
Now, the tuff one : $G(x,y,z)$. Remember, we want to find some factors like those in $H(x,y)$. So, we have to use our imagination...
\begin{align*}
G(x,y,z) &= -11xy-xz \\
&= -3x(3y+z)-2xy+2xz \\
&= -3x(3y+z)-2x(y-z) \\
\end{align*}
Finish, with all the pieces together.
\begin{align*}
F(x,y,z) &= 3x^2 + G(x,y,z) + H(y,z) \\
&= 3x^2-3x(3y+z)-2x(y-z)+2(y-z)(3y+z) \\
&= (3y+z)[2(y-z)-3x]-x[2(y-z)-3x] \\
&= [2(y-z)-3x][(3y+z)-x] \\
&= (3x-2y+2z)(x-3y-z)
\end{align*}
A: The most systematic way is to complete the squares and then factor as a difference of squares, but I dont have the energy to do that right now, maybe someone else would like to post such a solution. 
Another way is treat it like ordinary factorisation, say in variable $x$,
$$3x^2-(11y+z)x+6y^2 -4yz+2z^2$$ and if we factor the non $x$ terms we have 
$$6y^2 -4yz+2z^2=2(z-y)(z+3y)$$
then a little trial ane error gives
$$[3x+2(z-y)][x-(z+3y)]$$
