`U=(x+3y)^3-(x+3y)(x^2-3xy+9y^2)-2x(x-2)^2`
`=x^3+9x^2y+27xy^2+27y^3-(x^3+27y^3)-2x(x^2-4x+4)`
`=x^3+9x^2y+27xy^2+27y^3-x^3-27y^3-2x^3+8x^2-8x`
`=(x^3-x^3-2x^3)+9x^2y+27xy^2+(27y^3-27y^3)+8x^2-8x`
`=-2x^3+9x^2y+27xy^2+8x^2-8x`
`=-2.1^3 +9.1^2 . 2+27 . 1 . 2^2+8 . 1^2-8.1`
`=-2+18+108+8-8`
`=124`
Vậy `U=124` khi `x=1;y=2`