a. If a friend says his grade is 12 points lower than your grade, it means your grade is greater than his grade, if your grade is represented by x, then the grade of your friend can be found by subtracting 12 from your grade, this can be written as:
b. If x represents your friend's grade instead of yours, then to find your grade you need to add 12 to his grade, it can be represented as:
The correct answers are B. and D.