hello!
In order to balance the equation, John should multiply the second side of the equation by s.
Why?
Remember the Golden Rule of Algebra:-
So if you multiply 1 side of the equation by s, you multiply the second side by s.
Please tell John about the Golden Rule of Algebra ;)
note:-
Hope everything is clear; if you need any explanation/clarification, kindly let me know, and I will comment and/or edit my answer :)