hey guys, I am stuck on this question: "The Remainder when x^3-2x^2+ax+5 is divided by x-3 is twice the remainder when the same expression is divided by x+1. Find the value of the constant a." I know the answer is 4 but i dont know how to get there.

I have started it off but dont know what to from then on...

I worked out f(3) to be -14=3a ...and f(-1) to be a=2

Am I on the right lines? and what do I do from then on? this will really help me out