Well that's fucked..? df(x) / xf'(x) looks pretty hard..
If you mean with respect to x, then that's also fucked because you need an equation with r in terms of x
If you meant differentiate with respect to r, then try this:
f(r)= [(r-6)^3 +1 / (r-6)^3 + 1] -[2/(r-6)^3+1]
= 1 -2/(r-6)^3+1...