Translation:I think she found a new exercise.
In legal or buisness language they say "esercizio (commerciale)", a "commercial operation" but is not used in everyday speaking.
It exists also "esercitare una professione" that is related and again a quite formal language.
If you consider lei as courtesy form you set the high register required for this tipe of translation but is still a quite improbable sentence. But a few are not on Duolingo :)
There are certain adjectives that change meaning a little depending on if they are before or after the noun. This webpage gives examples:
"nuova" could be used in either position, depending on what you actually meant. In this particular exercise, Duolingo put it after the noun.