Hi!

I have this problem in my book that I really have trouble with:

Given a vector space V and a subspace U of V. Prove that phi of U [phi is the natural isomorphism from U to U** given by phi(u)(f) = f(u)] is equal (not just isomorphic!) to the double annihilator of U. Given G in the double annihilator, find u in U such that G=phi(u)

Any help would be greatly appreciated!!