12
To find the answer you multiply 8x5, since there's five numbers in the data set. The answer would be 40. Then, add all the numbers in the data set that you have. (3,6,9,10) and the answer would be 28. Then subtract 40-28=12. You can check your answer by adding 3,6,9,10,12 then dividing by 5, giving you the mean, 8.
Hope this helps!