What happens if you fail a quarter in high school, does that credit not count anymore ? and do i have to retake the class if i pass the other 3 quaters

Sally is making a model of a magnesium atom with an atomic mass number of 24 for her chemistry class. she has foam balls for the protons, neutrons, and electrons. she has added 6 neutrons to her model so far. how many more neutrons does she need to add to complete her neutral atom of magnesium?