the median is 19.5
Explanation:
To find the median you list all numbers in number order, which you already have and find the center number.
13, 14, 17, 18, 21, 23, 26, 28
/ / / 19.5 / / /
If there are two numbers left you can add then together and divide by 2 to find the median.
18+21=39
39/2= 19.5