Mean is all the numbers together in total so in order to get the mean you just add up all the numbers then divide by the total amount of numbers in that set.
Median in order to find the median you rearrange the numbers least from greatest and find the average from the two numbers in the middle of the set 2,2,2,2,2,4,5,5 21, in this case 2 because it's the number in the middle.
Mode in a set of numbers the mode is the most occurring of numbers 2,2,2,4,5,21,2 2,5 which would be 2 in this case.
Hope this helps.