Solve for r.

f/r^2 + a  = n + k        subtract "a" from both sides

f/r^2 =  n + k  - a         multiply both sides  by r^2

f  = r^2 ( n + k  -a )     divide  both sides by (n + k  -a)

f / ( n + k - a)  = r^2       take  both roots

±√ [ f / ( n + k -a) ]  =  r   ⇒  first  answer