$\displaystyle q(x1,x2,x3,x4)=2x_1x_4-6x_2x_3$

my prof said to me

that when i dont have squares the use this substitution

x1=x1'-x4'

x4=x1'+x4'

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

thanks