`19.`

`5x^2 ( x-1) + 10xy( x-1) - 5y^2 ( 1- x)`

`=5x^3-5x^2+10x^2y-10xy-5y^2+5xy^2`

`=5(x^3−x^2+2x^2y−2xy+xy^2−y^2)`

`=5[x^2(x−1)+2xy(x−1)+y^2(x−1)]`

`=5(x^2+2xy+y^2)(x−1)`

`=5(x+y)^2(x−1)`

`a^2 + b^2 - x^2 - y^2 + 2ab - 2xy`

`=(a^2 + 2ab + b^2)-(x^2+2xy+y^2)`

`=(a+b)^2 - (x+y)^2`

`=(a+b+x+y)(a+b-x-y)`

_____________________________

`20.`

`x^2 -x - 56`

`=x^2+7x−8x−56`

`=x(x+7)−8(x+7)`

`=(x−8)(x+7)`

`x^2 - 17x + 72`

`=x^2−8x−9x+72`

`=x(x−8)−9(x−8)`

`=(x−9)(x−8)`

`2x^2 + 3x - 27`

`=2x^2+9x−6x−27`

`=x(2x+9)−3(2x+9)`

`=(x−3)(2x+9)`