The mean, or average, is equal to the sum of all the miles divided by the number of terms you added together. We have 4 terms that will be added, but are missing one term. We will solve for the missing term by setting up an equation for the average and solving for x, the number of miles you must run on Thursday.
3 = (2 + 3 + 4 + x) / 4
12 = 2 + 3 + 4 + x
12 = 9 + x
x = 3 miles
Hope this helps! :)