Do I need to be a math expert to understand this?

This joke has two big parts the first part is the child doesn't understand the assignment, she misunderstands the comment about dividing X by 2, and she produces the variable C because of her handwriting. In some higher level mathematics such as calculus the variable C represents a range of numbers the exact one of which is unknown. This range includes the number 1 the answer for the specific problem. That leads us to the second part of the joke which is the child writing something that is mathematically well beyond her level (at graduate lever) and the teacher assessing it as the child being exceptionally intelligent and ready for graduation. It is a humorous juxtaposition of simple and profound and the results there of.