input: Write a python function to add two matrices.
  assert add_matrix([[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_matrix([[1,4,7], [2,5,8], [3,6,9]], [[9,1,2], [5,3,4], [1,8,7]]) == [[10, 5, 9], [7, 8, 12], [4, 14, 16]]
  assert add_matrix([[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_matrix(matrix1, matrix2):
      return [[matrix1[i][j] + matrix2[i][j] for j in range(len(matrix1[0]))] for i in range(len(matrix1))]