Particle Dynamics and Time Problem

When a particle fall through the air, its initial acceleration a=g diminishes until it is zero, and thereafter it falls at a constant or terminal velocity vf. If this variation of the acceleration can be expressed as a=(g/v^2f)(v^2f-v^2), determine the time needed for the velocity v<vf. Initially the particle falls from rest.

 

find the time for the velocity to become v<vf. (V less than Vf)

 

make the forces and gravitation = to eachother? can you help me please?

 

dv/dt = (g/v^2f)(v^2f-v^2)? like that? how? can you please show me