I currently have the equation:
$$1=\frac{x^2}{75}+\frac{y^2}{56.25}$$

 for my ellipse and I have found the values such as the foci($2.5\sqrt{3}, 0$) and ($-2.5\sqrt{3}, 0$) with the eccentricity of 0.5. I have translated this into the form of $r=\frac{ep}{1-ecos(x)}$
$\Rightarrow r=\frac{3.75}{1-0.5cos(x)}$

 I understand that this is the correct shape for my ellipse but it does not have the original center at the origin, which is the reason to why I am asking this question. How can I show algebraically rearranging my Cartesian formula to polar formula and still maintain my center at the origin?