First you add up all of the numbers you've been given and then you divide that sum by the number of terms you added. For example, if you were given 21, 45, 58, 66, 67, 80, and 92 you would add them all together and get 429. Then you would divide 429 by 7 (because that is the number of numbers we added together) and you would get about 61.29.
So basically
Step 1: add all numbers together
Step 2: divide sum by the number of terms.