`16. (3x - 5)(7 - 5x) - (5x + 2)(2 - 3x)`
`= 21x - 15x² - 35 + 25x - 10x + 15x - 4 + 6x`
`= -15x² + 57x - 39.`
`17. (x² - 3x)(2x³ - 4x²)`
`= 2x^5 - 4x^4 - 6x^4 + 12x³`
`= 2x^5 - 10x^4 + 12x³`
`18. (3x² + 5x - 2)(2x² - 4x + 3)`
`= 6x^4 - 12x³ + 9x² - 10x³ - 20x² + 15x - 4x² + 8x - 6`
`= 6x^4 - 23x³ - 15x² + 23x - 6.`
`19. (2x + 3)(x² - 4x + 3)`
`= 2x³ - 8x² + 6x + 3x² - 12x + 9`
`= 2x³ - 5x² + 6x + 9`
`20. (x - 5x²)(x² + 6x + 4)`
`= x³ + 6x² + 4x - 5x^4 - 30x³ - 20x²`
`= 5x^4 - 29x³ - 14x² + 4x.`
`21. (a³ - a + 3)(2a + a² + 1)`
`= 2a^4 +a^5 + a³ - 2a² - a³ + a + 6a + 3a² + 3`
`= a^5 + 2a^4 + a² + 7a + 3.`
`22. (7x² + 3x + 1)(2x² - 4x + 3)`
`= 14x^4 - 28x³ + 21x² + 6x³ - 12x² + 9x + 2x² - 4x + 3`
`= 14x^4 - 22x³ + 11x² + 5x + 3`
`23. (4x² - 7x + 2)(x² + 2x - 1)`
`= 4x^4 + 8x³ - 4x² - 7x³ + 14x + 7x + 2x² + 4x - 2`
`= 4x^4 + x³ - 2x² + 18x - 2`