input: 
  def averageMatrix(matrix):
      sum = 0
      for row in matrix:
          for e in row:
              sum += e
  
      return sum / (len(matrix) * len(matrix[0]))
  
  a = [[3,2,6],
       [8,5,9],
       [4,7,1]]
  
  print(averageMatrix(a)) # 4.77
  
output:     count = 0
      for row in matrix:
          count += len(row)
      return sum / count