`a)5x^2(3x^3-2x+1)`
`=15x^5-10x^3+5x^2`
`b)(1-x)(1+x)`
`=1^2-x^2`
`=1-x^2`
`c)(x+3)(x^2-3x+9)`
`=(x+3)(x^3-3x+3^2)`
`=x^3+3^3`
`=x^3+27`
`d)(x^2-1)(2x^2-x+3)`
`=2x^4-x^3+3x^2-2x^2+x-3`
`=2x^4-x^3+(3x^2-2x^2)+x-3`
`=2x^4-x^3+x^2+x-3`
$\text{@ThanhTruc2008}$