you have to change the denominators to the same thing by multiplying. You could do 21. To get 7 to 21, you have to multiply by 3. Whatever you do to the bottom, you have to do to the top, 5 x 3 equals 15. So 5/7 is equal to 15/21. To get 3 to 21, you have to multiply by 7. Again, same thing, whatever you do to the bottom, you have to do to the top. 2 times 7 equals 14. So 2/3 is equal to 14/21. Then you subtract 15/21 - 14/21. When you subtract fractions with the same denominators, you ONLY subtract the numerators. 15 -14 equals 1. So the answer is 1/21.