If we call Jackie's apples a, then Mark has a - 4 apples. Therefore, together they have a + (a - 4) apples, and we know that they have a total of 12 apples so we can write a + (a - 4) = 12 to represent the situation. On the other hand, if Mark has a apples, then Jackie has a + 4 apples, so we can also write a + (a + 4) = 12 to represent the situation as well. Therefore, both of the equations would work. Hope this helps!
