What is 25 raised to the negative 3/2 power?
An exponent of the form 1/n for a number n is taken to mean taking the nth root of the number. In this case, we're just working with n=2, which means a square root and should be familiar to you.
253/2 can be easily rewritten by taking the square root first. I'd rather not deal with cubing 25.
253/2 = 53 = 125.