Hopf Invariant (Differential Topology)

Hello everyone, this is a problem out of Milnor (Topology from a Differentiable viewpoint) I have been assigned to solve. First, definition of the "Linking number"

Still with me? Ok, you can define the "Hopf Invariant" in the following way:

So I need to show three things (and any help on ANY them is appreciated :-) )

a) The Hopf Invariant is locally constant as a function of y.

b) If y and z and regular for g also and then

c) Prove depends only on the homotopy class of f and does not depend on the choice of y and z.

Basically, prove H(f) is a well-defined invariant. Any thoughts?