I am reading my differential equations book and it is going over the differential inverse operator and more specifically this case where $y_p=\frac{1}{D-a_0}(bx^k)$. So then they do these two steps and I do not know how they did the second one. $$=\frac{1}{-a_0(1-\frac{D}{a_0})}(bx^k)$$ $$=-\frac{1}{a_0}\left[1+\frac{D}{a_0}+\frac{D^2}{a_0^2}+\frac{D^3}{a_0^3}+...+\frac{D^k}{a_0^k}\right](bx^k)$$
I don't know how the two are equal. I feel like it might just be something I forgot from my last math classes. The book says it is the series expansion of the inverse operator by ordinary division. I do not know what it means that the series expansion is by "ordinary division"
Can someone please explain to me what they did there in that step and what is the "ordinary division" part or maybe point me to some place I can learn about it?