You have to provide guess values for the three variables you'd like to vary and when you call "maximize" you have to use the function name as first argument.
The results vary significantly if you change the guesses!
In your file your wrote "Define p, q as constraints".
If you want r dependent on specific values of p and q, you could do this as follows:
But if this is what you are looking for you could as well use the derivative and the numeric "root" function 😉
