Need help using min_max

Dear Eclipse Users,

I'm having a very beginning level problem that I have not been
able to solve by myself: I cannot correctly implement min_max
or any other optimazation predicate.  I don't understand 
how it operates, and I have a suspicion that my code is "binding" 
variables involved in the optimization that need to be "free".  
(Can you tell I'm also very new to Prolog?)

I can include some small samples of code, if that would be helpful. 
 
Thanks for any clues you can offer me, 
*Nira Brand