Can anyone please help me???

It asks to find the power series representation and radius of convergence for

f(x) = ln(a - x) for a>0

As an answer I got the series 1 to infinity of

__x^n___

a^n+1

I don't know if it is right and the radius of convergence I got was a but I don't know if that is right

f(x) = __x^3___

(1-4x)^2

I have no idea how to do this one...

Please help me