input: Write a function to find the longest line in a given string with multiple lines.
  assert find_longest_line('Sentence 1\nSentence 2\nSentence 3 (longest sentence)\nSentence 4')=='Sentence 3 (longest sentence)'
  assert find_longest_line('Line one\nLine two is longer\nShort line')=='Line two is longer'
  assert find_longest_line('First line\nSecond line\nThird line\nFourth and the longest line')=='Fourth and the longest line'
output: def find_longest_line(s):
      lines = s.split('\n')
      longest_line = max(lines, key=len)
      return longest_line