Population decreases by 15.
Birth rate means more population (positive), death rate means less population (negative), immigration means more (positive), and emigration means less (negative). So, just add and subtract the appropriate numbers:
[tex]35\ people\ born-7\ people\ die+4\ people\ enter-17\ people\ leave=\\-15\ people[/tex]
Negative 15 people means that the population decreases by 15.
