Physics help

The frequency f of oscillation of a mass m on a spring of spring constant k is given by f=1/2pi * sqrt(k/m).
Find an expression for the rate of change of f with k in terms of f and k , assuming that the mass m is constant.
The following may be useful: f, k, pi.
Differentiating the expression, you get df/dk = 1/[4pi*sqrt(m)] * k^-1/2,
One of the hints said that it would be useful to consider defining f = C*sqrt(k), where C = 1/[2pi * sqrt(m)] and then to eliminate it from the resulting expression for the derivative, but I am still very confused as to how you go about this.
Thanks for any help.

Assuming I understand your maths right, your derivative looks about right. They want you to express it in terms of f, k and pi (so not m) so write the deriviative in terms of f (sub the original expression into it).
As the hint says (and is fairly common in isaac physics questions), lumping all the constants together into a new one often simplifies the analysis / makes clear about what arguments to do.