To solve this problem you must apply the proccedure shown below:
 1. You have the following information given in the problem above:
 - Mount Mckinley is 20,320 feet above sea level. 
 - Mauna kea is 13,796 feet above sea level.
 2. Then, you only need to substract both quantities to solve the exercise, as following:
 

 Therefore, as you can see, the answer is: 6,524 feet.