Hi! Here I'm again!

If $\displaystyle 5^{3a} = 64$ , so $\displaystyle 5^{-a}$ is?

I've tried change the base, ($\displaystyle 64 = 2^6 $), for example... But I coudn't get a result, because I don't know what to do with the $\displaystyle 5^3 = 125 $ and the $\displaystyle 5^a$ , or how to transform they on $\displaystyle 2$

>>>

$\displaystyle

5^{3a} = 64$

$\displaystyle 5^3\times5^a = 2^6

$

This is the way?

How can I do this?!