First,
A negative exponent means that the base (coefficient) for that exponent will change to a fraction with 1 over that number. (p^-15 becomes 1/p^15) This is like an inverse function for a graph.
However, since p^-15 is in the denominator, we take the inverse of that inverse XD, as below:
1/p^-15 ->> 1/(1/p^15) ->> p^15
The Inverse of an inverse cancel each other out!
Good luck!!