Since the problem states "a number" and again "a number", I assume it;'s the same number. Let the number be x.
12x - 9x = 3x
The difference of twelve times a number and nine times a number is three times the number.
12x-9y
Difference means subtraction, so the difference between 12 times a number is known as 12x because we do not know what is being multiplied by 12. Same with 9 times a number. Since 12 and 9 have two numbers that we don't know the value of, we have to change one of them to a different variable because we do not know if it is the same number or not. So it would be 12x-9y, or any letters you choose.
