hello!
In order to find the reciprocal of a number, we should just flop it over.
If we're asked to find the reciprocal of a fraction, then the numerator & denominator of the fraction switch places.
If we're asked to find the reciprocal of an expression, then we also flop it over, like so:-

Explanation of steps:-
First, I turned the expression into a fraction by adding a denominator of 1.
Then, I flopped it over and found the reciprocal.
Hence, the reciprocal of u²-s² is:-

note:-
Hope everything is clear; if you need any explanation/clarification, kindly let me know, and I will comment and/or edit my answer :)