I don't want to just give it away, so here is a hint: look at a configuration of y that is not arranged in increasing order of magnitude and look at the product xy. Can you make it larger by changing the configuration y in a small way?
x and y are row vectors having non-negative elements. The elements of x are arranged in the increasing order of magnitude. The positions of elements of y can be re-arranged. How can I show that xy' is maximum when the elements of y are also arranged in the increasing order of magnitude?
I don't want to just give it away, so here is a hint: look at a configuration of y that is not arranged in increasing order of magnitude and look at the product xy. Can you make it larger by changing the configuration y in a small way?