`2xy^2 (xy+3x^2y-2/3xy^3)`
`=2xy^2 . xy+2xy^2 . 3x^2y-2xy^2 . 2/3xy^3`
`=2x^2y^3 + 6x^3y^3 - 4/3x^2y^5`
$$$$
`3x(2x^3-4x)`
`=3x.2x^3-3x.4x`
`=6x^4-12x^2`
$$$$
`3/5x^3y^4 (7x^4+5x^2y-10/21x^4y^3-y^4)`
`=3/5x^3y^4 . 7x^4+3/5x^3y^4 . 5^2y-3/5x^3y^4 . 10/21x^4y^3 - 3/5x^3y^4 . y^4`
`=21/5x^7y^4+3x^5y^5-2/7x^7y^7 - 3/5x^3y^8`