FP1 Numerical solutions help

Well as it doesn't tell you I would choose a random (within reason) negative number. If you have a calculator that has the function to solve a cubic equation, it may be a good idea to check to give you a hint of what to choose (so you can make an amazingly good guess :P ), if not as it states that there is only one answer, i would go for a suitably negative number like -5 or -3 as they give you no idea of what they would prefer.