What is $25^k$ + $5^k$

Question

This is an extremely simple problem, but I can't find an example anywhere for some reason.

I know that $30^k$ is not correct

But I have no idea what else makes sense.

Answer 1

This is about a simplified as you can get. You can also write it as $5^k(5^k+1)$. if you prefer that. There's no simple formula like $a^k$.