OK I think I understood your question:
Given a vector v, how to find the unit vector at the same direction?
It is v/|v|
the question says that shark finds preys through smell and electrical impulse. and f(x,y,z) indicates the electrical pulse in the water at position (x,y,z) and shark senes gradient f <12,-20,5>, in which direction should the shark swim to find its prey.