Series expansion of a function up to linear terms

Question

I have the following function: $$f=\Sigma/\Delta$$ where $$\Sigma=r^{2}+a^{2} \cos ^{2} \theta$$ $$\Delta=r^{2}-2 M r+a^{2}-\frac{k}{3} r^{2}\left(r^{2}+a^{2}\right)$$ and I want to obtain the expansion of $f$ up to linear terms in $M/r$ and $k r^2$. I could not get the form of the right answer which is $$f=1+\frac{2M}{r}-\frac{kr^2}{3}$$ Does anyone have an idea as to what I am missing here? Thanks