The commutative property is a property of <em>addition</em>, not of <em>subtraction</em>. It can work in a subtraction problem if you keep the sign with the number, so you're adding positive and negative numbers.
Claudia is making 12 serving for 5 people and she needs to make enough for 10. which means that she needs to make 12 + 12 = 24 servings for 10 people. dessert mold holds 8 servings. 24 / 8 = 3 the minimum number of these molds is 3.