I have been presented with the following question :

The polynomial 

$$f(x) = x^3 - 2x^2 +ax + b$$

satisfies the following :

a) It is divisible (x-1)

b) it leaves a remainder of -24 when divided by (x+3)

I have no idea where to begin with this, I have tried long division but I get stuck when ax becomes the dividend. I have also tried substitution and rearanging the equation but im not getting anywhere. If anyone could show me how to complete this question, or shed any light on the steps I would be extremely grateful.