I need to modify the last statement in the item_add_click function so it calls the update_display function by using the call method of that function
What do you mean by "the method of a function"? There is no method defined there, and, anyway, methods can be defined as being properties of an object, or there is no object created there becuase you did not use that function as a constructor.
Can you detail a little bit your aim, please? I confess I did not understand which, in fact, is your problem.