I thought you needed "de" between the adjective and the infinitive to get this meaning. From my research, "Es + adjective + de + infinitive" is "It is [adjective] to [verb] and "Es + adjective + infinitive" is "[Verb]ing is [adjective]. On a prior lesson in this module "Es imposible de saber" was the correct answer.
Both are possible. http://forum.wordreference.com/showthread.php?t=211005 provides a great explanation if you can understand Spanish well enough. Basically, it depends on where the complement is placed. Consider the "es imposible de saber" example. The hidden (algo) is the compliment, so "(algo) es imposible de saber" is correct as well as "es imposible saber (algo)."
Some verbs have built in prepositions. "salir" = "to go out". I assume an "a" would be redundant here. http://www.cliffsnotes.com/foreign-languages/spanish/spanish-i/prepositions/preposition-use-with-verbs