Solve for R

f/(r ^ 2)  = n + k - a     after subtracting  'a' from both sides of the equation

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

r = +- sqrt ( f/(n+k-a) )       (and as WIJ pointed out   n+k-a cannot equal 0)

$r=\sqrt{\frac{f}{n+k-a}}, r=-\sqrt{\frac{f}{n+k-a}};\quad \:a\ne \:n+k$