Hi
It is unclear what your problem with this equation is. At a first glance, I would say, that if you have read at least the manual up to the chapter “Model and Solve statement”, you should be capable of writing this equation.
Furthermore, as noted in the “Rules” of this forum, the chance of getting an answer to your question is increased dramatically if you add the code you are working on and which produces an error or at least the code people could use to give you answer.