Hey guys I am a bit stuck on how to do this question. Could you please go through step-by-step on how to get the answer! Thank You!

"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 constant value of a."

I started off like: f(3)=2(x-1) but I am not sure if that is correct or not because I didn't get the right answer. Am I along the right lines but there are more steps or am I off the line completely? Once I can do this question I can answer some of the other questions which is a similar style to this.

Thanks

DantheMaths