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

 

f/(n+k-a)  = r^2             take sqrt of both sides

 

+- ( f/(n+k-a) ) = r