`***`Lời giải`***`
`( x-y+6)(x+y-6)`
`=x(x+y-6)-y(x+y-6)+6(x+y-6)`
`=x^2+xy-6x-xy-y^2+6y+6x+6y-36`
`=x^2-y^2+(xy-xy)+(-6x+6x)+(6y+6y)-36`
`=x^2-y^2+12y-36`
`(y+2z-3)(y-2z-3)`
`=y(y-2z-3)+2z(y-2z-3)-3(y-2z-3)`
`=y^2-2yz-3y+2yz-4z^2-6z-3y+6z+9`
`=y^2-4z^2+(2yz-2yz)+(6z-6z)+(-3y-3y)+9`
`=y^2-4z^2-6y+9`
`(x+2y-3z)(2y+3z-x)`
`=x(2y+3z-x)+2y(2y+3z-x)-3z(2y+3z-x)`
`=2xy+3xz-x^2+4y^2+6yz-2xy-6zy-9z^2+3xz`
`=-x^2+4y^2-9z^2+(6yz-6zy)+(2xy-2xy)+(3xz+3xz)`
`=-x^2+4y^2-9z^2+6xz`