"Impossible to do another way"
Translation:Impossible de faire autrement
I think it is because this is an impersonal sentence without a subject. If you were talking about someone or something in particular that was "impossible to do" then it would be "à faire":
Il est impossible de faire (no subject)
Il y a des choses à faire (subject: des choses)
...I got this wrong, too. :( I recall someone before saying that in French, infinitives sometimes require á/de depending on whether they are transitive (verbs that require an object) or intransitive (verbs that don't require an object). However, some verbs that are transitive in English are intransitive in French and vice versa, which makes this particularly frustrating for English speakers.
Furthermore, I believe that there is no exact rule on whether one should use á as opposed to de with the verb, it has to be memorized. :(
This should help:
First link has a list of (I'm not exactly sure if it's complete) verbs that require the "á" preposition, and the second one with "de."