input: Write a python function to add two 3x3 matrices.
  assert add_matrices([[1, 2, 3], [4, 5, 6], [7, 8, 9]], [[9, 8, 7], [6, 5, 4], [3, 2, 1]]) == [[10, 10, 10], [10, 10, 10], [10, 10, 10]]
  assert add_matrices([[0, 0, 0], [0, 0, 0], [0, 0, 0]], [[1, 2, 3], [4, 5, 6], [7, 8, 9]]) == [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
  assert add_matrices([[1, 1, 1], [1, 1, 1], [1, 1, 1]], [[2, 2, 2], [2, 2, 2], [2, 2, 2]]) == [[3, 3, 3], [3, 3, 3], [3, 3, 3]]
output: def add_matrices(matrix1, matrix2):
      result = [[matrix1[i][j] + matrix2[i][j] for j in range(3)] for i in range(3)]
      return result