I have the answer to this question,, but i'd like to know how to do it. I need help finding z. Any help would be great. Thanks.

Question

Children and McDonald�fs In a Sports Illustrated for Kids survey of 603 children, 43%
preferred McDonald�fs for fast food. The next highest rating went to Taco Bell, preferred by
13% of the children. An advertising executive claims that McDonald�fs is preferred by less than
half of all children. Test that claim using a 0.05 significance level.

I think I use the following formula:

z = p-hat - p / �ã pq/n