In this task you will be given a list of numbers and you need to find the mean (average) of that list. The mean of a list can be found by summing every number in the list then dividing the result by the size of that list. The output should be rounded to 3 decimal places.
[91.594, -42.447, 221.845, 129.556, 34.139, -39.215, -68.586]