input: 
  def averageMatrix(matrix):
      """ Calculate the average of elements in a 2-dimensional array.
      >>> a = [[3,2,6], [8,5,9], [4,7,1]]
      >>> averageMatrix(a)
      4.777777777777778
      """
      sum = 0
      for row in matrix:
          for e in row:
              sum += e
  
      return sum / (len(matrix) * len(matrix[0]))
  
output:     sum = 0
      for row in matrix:
          for e in row:
              sum += e
  
      return sum / (len(matrix) * len(matrix[0]))