Have you provided answers to this so that we choose from "which of the following"?
It would be 5^-x/(2^-x) when the exponent is distributed.
The negative sign in the exponent would make the numerator and denominator change places, so it becomes
2^x/5^x
which you could call
(2/5)^x
It might help to see the actual choices, but I hope that this simplification helps!

Phil S.
03/18/20
Hadassah J.
f(x)=−(52)^−x f(x)=−(52^)x f(x)=x^−25 f(x)=2^−x/5 f(x)=(2/5)^x f(x)=−(2/5)^x03/18/20