$\text {b,}$
`(x-2y)(x^2y^2-xy+2y)`
`= x^3y^2-x^2y+2xy-2x^2y^3+2xy^2-4y^2`
$\text {c,}$
`(2)/(5)xy(x^2y-5x+10y)`
`= (2)/(5)x^3y^2-2x^2y+4xy^2`
$\text {d,}$
`(2)/(3)x^2y(3xy-x^2+y)`
`= 2x^3y^2-(2)/(3)x^4y+(2)/(3)x^2y^2`
$\text {e,}$
`(x-y)(x^2+xy+y^2)`
`= x^3-y^3`
$\text {g,}$
`((1)/(2)xy-1)(x^3-2x-6)`
`= (1)/(2)x^4y-x^2y-3xy-x^3+2x+6`
$\text {@lamtung2007}$