Definition: 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.
Input: [-16.878, -94.397, -58.691, 131.743, 218.247, 140.732, 133.169, -30.788, 42.957]

51.788