Question 180840
x(x+a)x+b) = x^3 - 4x^2 - 12x
x^3 + x^2 (a+b) + abx = x^3 - 4x^2 - 12x

Compare each term on both LHS & RHS

(a+b) = -4  &   ab = -12

substitute a= 2, b= -6; which satisfy the expression.