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.
[-16.859, 65.309, -33.432, -57.702, 3.867, -28.83, -34.842, 170.229, 31.362]