To make x-y reasonable, you must make x the same value as y. It is the same as y-x. But a different variable order. For example x= 6 and y=9, and x - y is unreasonable, to make it reasonable, you must add 3 more to x to make it the way of y - x. So, your answer is just like 9-9 or 10-10 or any other subtraction equation with equal value, you don’t have to know what the variables equal, you just have to take an example and try it, that’s all you need to do to get out of the tricky problems. Hope this helps and have a great night of sleep :))