$\displaystyle q(x1,x2,x3,x4)=2x_1x_4-6x_2x_3$
my prof said to me
that when i dont have squares the use this substitution

so then i woulde have squares and there would be other x1' x4' to link to that square
to complete into (a+b)^2

but here i dont have that

please dont present me something totaly new
i just want the fixup to my prof's method to solve it