`a)\ x^{2}-x-20=(x^{2}-5x)+(4x-20)=x(x-5)+4(x-5)=(x-5)(x+4)`
`b)\ x^{2}-x-56=(x^{2}-8x)+(7x-56)=x(x-8)+7(x-8)=(x-8)(x+7)`
`c)\ x^{2}+4x-21=(x^{2}-3x)+(7x-21)=x(x-3)+7(x-3)=(x-3)(x+7)`
`d)\ 7x^{2}+2x-9=(7x^{2}-7x)+(9x-9)=7x(x-1)+9(x-1)=(x-1)(7x+9)`
`e)\ 2x^{2}-3x+1=(2x^{2}-2x)-(x-1)=2x(x-1)-(x-1)=(x-1)(2x-1)`
`f)\ 5x^{2}-6x-11=(5x^{2}+5x)-(11x+11)=5x(x+1)-11(x+1)=(x+1)(5x-11)`
a: =(x-5)(x+4)
b: =(x-8)(x+7)
c: =(x+7)(x-3)
d: =(x-1)(7x+9)
e: =(x-1)(2x-1)
a: =(x-5)(x+4)
b: =(x-8)(x+7)
c: =(x+7)(x-3)
d: =(x-1)(7x+9)
e: =(x-1)(2x-1)