`2x^3 - x^2 - 13x - 6`

`=2x^3 + 5x^2 - 6x^2 + 2x - 15x - 6`

`=(2x^3 + 5x^2 + 2x) - (6x^2 + 15x + 6)`

`=x(2x^2 + 5x + 2) - 3(x^2 + 5x + 2)`

`=(x-3)(2x^2 + 5x + 2)`

`=(x-3)[(2x^2 + 4x) + (x+2)]`

`=(x-3)[2x(x+2)+(x+2)]`

`=(x-3)(2x+1)(x+2)`

`8x^2 + 30x + 7=0`

`=> 8x^2 + 2x + 28x + 7=0`

`=> (8x^2 + 2x) + (28x+7)=0`

`=> 2x(4x+1)+7(4x+1)=0`

`=> (2x+7)(4x+1)=0`

`=> [(2x+7=0),(4x+1=0):}`

`=> [(2x=-7),(4x=-1):}`

`=> [(x=-7/2),(x=-1/4):}`

Vậy `x in {-7/2 ; -1/4}`