`a)`
`(2x-1)x+x^2-2(x-3)`
`=2x^2-x+x^2-2x+6`
`=(2x^2+x^2)-(x+2x)+6`
`=3x^2-3x+6`
`b)`
`x(2x^2-3x)-(x+1)(x^2-2)`
`=2x^3-3x^2-(x^3-2x+x^2-2)`
`=2x^3-3x^2-x^3-x^2+2x+2`
`=(2x^3-x^3)-(3x^2+x^2)+2x+2`
`=x^3-4x^2+2x+2`
`c)`
`3x(x-1)+x^2(x-2)+3(x-1)(x+1)`
`=3x^2-3x+x^3-2x^2+3(x^2-1)`
`=x^3+3x^2-2x^3-3x+3x^2-3`
`=x^3+(3x^2-2x^3+3x^2)-3x-3`
`=x^3+4x^2-3x-3`