Simplify.
(a) 2^3(5)^-2
=?
(b) 2^-3/a^4
= 2^3a^4
=8a^4????
2^3 = 8
5^-2 = 1/5^2 = 1/25
so, (a) = 8/25
You cannot simply disregard the negative sign of the exponent, nor willy-nilly move items from bottom to top. You have directly replaced 2^-2 with 2^3 and 1/a^4 with a^4.
2^-3 = 1/2^3 = 1/8
So, (b) = 1/(8a^4)