`1. (x+3)(x-1)-(x-7)(x-6)`
`= (x²+2x-3) -(x²-13x+42)`
`= x²+2x-3-x²+13x-42 = 15x-45`
`2. (-x-2)(x+1)-(x-5)(-x+1)`
`= (-x²-3x-2)-(-x²+6x-5)`
`=  -x²-3x-2+x²-6x+5= -9x+3`
`3. 5x(x-3)(x-1)-4x(x²-2x)`
`= (5x²-15x)(x-1)-(4x³-8x²)`
`= (5x³-5x²-15x+15)-(4x³-8x²)`
`= 5x³-5x²-15x+15-4x³+8x²`
`= x³+3x²-15x+15`