I've been building a puzzle game called Rule Hunter — each level gives you a

few example pairs and you have to figure out the hidden rule connecting them,

then apply it to solve the final case.

Here's one from the Normal difficulty:

3 → 9

5 → 25

8 → 64

7 → ?

What's the rule, and what's the answer?

(It's a free Android game, link in comments if anyone's curious — didn't want

to make this feel like an ad post.)