input: Write a python function to filter strings that do not contain the letter 'x'.
  assert filter_x(['abc', 'efg', 'X', 'xyz', 'xabc']) == ['abc', 'efg', 'X']
  assert filter_x(['hello', 'world', 'example', 'text']) == ['hello', 'world', 'example']
  assert filter_x(['xray', 'box', 'extra', 'max']) == []
output: def filter_x(strings):
      return [s for s in strings if 'x' not in s and 'X' not in s]