In The Phantom Tollbooth, what does King Azaz and the mathmagician tell Milo after he returns?

In The Phantom Tollbooth, Azaz is king of Dictionopolis and the Mathemagician, his brother, is the ruler of Digitopolis; they have never agreed on anything since the two princesses, Rhyme and Reason, were banished to the Castle in the Air. Milo sets out with his dog, Tock, and the Humbug to recover the princesses.
After many adventures, they convince the princesses to return with them but then must escape the pursuit of numerous evil forces. The brothers must join together to help them safely home. When they make it safely back, Milo is greeted as a hero. He insists that he succeeded only because of everyone’s help. The brothers then tell him the information they had withheld before he left, thinking that he would not have gone on the quest if he knew.

"It was impossible," said the king, looking at the Mathemagician.
"Completely impossible," said the Mathemagician, looking at the king. . . .
"Yes, indeed," they repeated together; "but if we'd told you then, you might not have gone—and, as you've discovered, so many things are possible just as long as you don't know they're impossible."
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