`a)`
`2x^2 +(x-1)(x+1) - 5x(x+1)`
` = 2x^2 + x^2 - 1 - 5x^2 - 5x`
` = (2x^2 + x^2 - 5x^2) - 5x - 1`
`= -2x^2 - 5x -1`
``
`b) 2x(x-3) - (x+2)(5x-4) + 15x^2`
`= (2x^2 - 6x) - (5x^2 - 4x + 10x -8) + 15x^2`
` = 2x^2 - 6x - 5x^2 + 4x - 10x + 8 + 15x^2`
` = (2x^2 - 5x^2 + 15x^2) + (-6x + 4x - 10x) + 8`
` = 12x^2 - 12x + 8`