input: def averageMatrix(matrix):
      """Calculate the average of all elements in a 2-dimensional array.
      >>> averageMatrix([[3,2,6],[8,5,9],[4,7,1]])
      4.77
      """
      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 round(sum / (len(matrix) * len(matrix[0])), 2)