input: def averageMatrix(matrix):
      """Compute and return the average of the elements of a 2-dimensional array.
      >>> a = [[3, 2, 6], [8, 5, 9], [4, 7, 1]]
      >>> averageMatrix(a)
      4.77
      """
      sum = 0
      for row in matrix:
          for e in row:
              sum += e
      return round(sum / (len(matrix) * len(matrix[0])), 2)
output:     sum = 0
      for row in matrix:
          for e in row:
              sum += e
      return round(sum / (len(matrix) * len(matrix[0])), 2)