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: [-28.295, -24.828, -66.96, -12.108, -96.597, 110.99, 172.86, -18.752, 157.717]

21.559