Answer:
In the given examples, your task is to find synonyms. Synonyms are words or phrases that have exactly or nearly the same meaning as another word or phrase in the same language.
Pent-up - Stored up
Pent-up energy is energy that has been held back and not released. Stored up, in this case, has the same meaning as pent up.
Millennia - Thousands of years
A millennium is a period of one thousand years. The plural of millennium is millennia, so the synonym, in this case, is thousands of years.
Cache - Hide
The verb to cache means to store something away in hiding or for future use. The verb it can be substituted with in this sentence is to hide.