input: Write a function to combine the lengths of two strings.
  assert combine_string_lengths('hello', 'world') == 10
  assert combine_string_lengths('Python', 'Programming') == 18
  assert combine_string_lengths('abc', 'defghij') == 10
output: def combine_string_lengths(str1, str2):
      return len(str1) + len(str2)