The compiler can't find that class. Most of your code is in the package "javacurrencyconverter;" FahrenheitToCelsius is not. It is in something called the "default package" because you've accidentally omitted the package line. Since you can't import the default package, the only code that can refer to would also have to be in the default package.
As you might imagine, the best fix is to add the missing package statement.