​ Solve for r.

$\frac{f}{r^2}+a=n+k\\ \frac{f}{r^2}=n+k-a\\ f=r^2(n+k-a)\\ \frac{f}{n+k-a}=r^2\\ r=\pm\sqrt{\frac{f}{n+k-a}}$

Your answer is correct!

Are a bunch of posters here on the same home-schooling program?  Because I am seeing a LOT of multiple posts of the EXACT same questions from DIFFERENT posters for the last few days....and they are posted with the EXACT same fonts !

Hmmmmm......