input: Write a function to print the multiplication table of 9.
  assert multiplication_table_of_9() == [
      "9 x 1 = 9",
      "9 x 2 = 18",
      "9 x 3 = 27",
      "9 x 4 = 36",
      "9 x 5 = 45",
      "9 x 6 = 54",
      "9 x 7 = 63",
      "9 x 8 = 72",
      "9 x 9 = 81",
      "9 x 10 = 90"
  ]
output: def multiplication_table_of_9():
      return [f"9 x {i} = {9 * i}" for i in range(1, 11)]