`a)2x(3x^2+2x-3)`
`=2x.3x^2+2x.2x-3.2x`
`=6x^3+4x^2-6x`
`b)(2x-5)(x^2-4x+21)`
`=2x.x^2-2x.4x+2x.21-5.x^2+5.4x-5.21`
`=2x^3-8x^2+42x-5x^2+20x-105`
`=2x^3-13x^2+62x-105`
`c)(x+3)(2x-1)-(3x+1)(x-3)`
`=x.2x-x.1+3.2x-3.1-(3x.x-3.3x+x-3)`
`=2x^2-x+6x-3-3x^2+9x-x+3`
`=2x^2+5x-3-3x^2+8x+3`
`=-x^2+13x`