I am assuming the -3 is an exponent.
y⁻³ and 5⁻³
If the exponent is a negative number, it simply means that the number is a denominator with 1 as the numerator.
y⁻³ is 1/y³
5⁻³ is 1/5³ = 1/125
The number is still raised to its exponential power but its placement is different.
5³ would be 125 or 125/1. It would be the numerator with 1 as the denominator.