Answer: -1
Raising one exponential expression to another exponent means you multiply the exponents together. We have 5*5 = 25 as the new exponent.
So we end up with (-1)^25 = -1
The rule I'm using is that -1 to any odd whole number power is going to result in -1. If the exponent was even, then -1 to that power would be 1.
So in short, the rule says
(-1)^n = -1 if n is an odd whole number
(-1)^n = 1 if n is an even whole number