The universal law of gravitation determines the amount of force exerted by a constant mass (M) on another constant mass (m), separated by a distance (r), is given by the expression F = ( -GmM )/r^2

Question

a) What mathematical expression characterises the instant rate of change of the force F in respect to the distance (r) separating the two masses moving towards each other?

I got dF/dt = 2GmM/r^3 dr/dt

b) What's the rate of change?

No idea what to do here, they don't give me any numbers.