You just have to do the opposite. So if you do 5+4=9 and you are not sure that is correct then you would do 9-5=4 or 9-4=5 if you get all of the same numbers that are in the question then it is right. It goes the same for fractions. I hope I helped.
You can check your answer with subtracting by adding the answer by either of the fractions you subtracted. For adding, you subtract the sum by one of the numbers you added. Hope I helped :)
