input: Write a python function to multiply two 3x3 matrices.
  assert matrix_mul3x3([[1, 2, 3], [4, 5, 6], [7, 8, 9]], [[10, 11, 12], [13, 14, 15], [16, 17, 18]]) == [[84, 90, 96], [201, 216, 231], [318, 342, 366]]
  assert matrix_mul3x3([[1, 0, 0], [0, 1, 0], [0, 0, 1]], [[1, 2, 3], [4, 5, 6], [7, 8, 9]]) == [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
  assert matrix_mul3x3([[2, 3, 4], [1, 0, 0], [0, 1, 1]], [[1, 2, 3], [4, 5, 6], [7, 8, 9]]) == [[40, 47, 54], [1, 2, 3], [11, 13, 15]]
output: def matrix_mul3x3(mat1, mat2):
      result = [[0, 0, 0], [0, 0, 0], [0, 0, 0]]
      
      for i in range(3):
          for j in range(3):
              for k in range(3):
                  result[i][j] += mat1[i][k] * mat2[k][j]
                  
      return result