`x^4+8x^3+14x^2-8x-15`
`=x^4-x^3+9x^3-9x^2+23x^2-23x+15x-15`
`=x^3(x-1)+9x^2(x-1)+23x(x-1)+15(x-1)`
`=(x-1)(x^3+9x^2+23x+15)`
`=(x-1)(x^3+5x^2+4x^2+20x+3x+15)`
`=(x-1)[x^2(x+5)+4x(x+5)+3(x+5)]`
`=(x-1)(x+5)(x^2+4x+3)`
`=(x-1)(x+5)(x^2+x+3x+3)`
`=(x-1)(x+5)[x(x+1)+3(x+1)]`
`=(x-1)(x+5)(x+1)(x+3)`