Answer:
=
= 0.0016
Explanation:
I'm assuming you mean
(which could also be written as 5^-4, for future reference).
Negative exponents actually mean you'd use the term in the denominator, taking away the negative portion. It's difficult to explain in words, and is easier to see.
=

Notice how the -4 changes to 4, and it becomes the denominator.
is also 625, so
is also 1/625, or the decimal version (0.0016).
I hope this helps!