This may help
http://en.wikipedia.org/wiki/Nth_root
a much more rigorous work can be found here:
http://www-history.mcs.st-and.ac.uk/HistTopics/Fund_theorem_of_algebra.html
applying simple logic that polynomial of degree two has two roots similarly polynomial of n degree has n no; of roots use brain it will simple crack iit
She needed proof. So we gave her links for that. By the way, they might also be given in some elementary text books.
If you find it in a text book, let me assure you, that textbook isnt elementary!