It might be an idea to state the question. It might also be an idea to listen to your teacher.
Your teacher's explanation was complicated because the problem involves 1 phase change, and 2 heat capacities. There is no simple way to solve this question and we must calculate stepwise the enthalpy change associated with the following transiions:
(i) Water as a liquid is cooled from
(ii) Water undergoes a phase transition at
(iii) Ice is cooled from
All of these processes may be separately calculated, given the appropriate thermodynamic quantities, and we need 3 of them, which you have not provided. I advise you to go back to your notes and follow your teacher's explanation.