Earlier quoted context omitted.
At -40 the two scales collide, you don't need to specify Celsius or Fahrenheit. Not far to go!
is F logarithmic or something.?
The conversion is F = (9/5) C + 32 .
If F = C then C = (9/5) C + 32 => (1-9/5)C = 32 => C = 32/(1-9/5) = -40.