input: Write a python function to convert Fahrenheit to Celsius.
  assert convert_f_to_c(32) == 0
  assert convert_f_to_c(212) == 100
  assert convert_f_to_c(98.6) == 37
  assert convert_f_to_c(0) == -17.77777777777778
output: def convert_f_to_c(fahrenheit):
      return (fahrenheit - 32) * 5 / 9