I suppose your first step would be to work out what to do with the arccos. So, where should that go? What does this tell you?

Secondly, you should work out that , and so that means the second proposition makes sense - the vectors are in the same direction. That is to say, they only differ by a scalar.

What you now want to do is prove that (why?). So, work out what is, and you will be done (why?)! (You will need the information you gleamed from my first line here).

Does that make sense?