First thing First. You must find have a common denominator. If you want to find it then you do this
3: 3 6 9 12 15 18 21
7: 7 14 21 28
Once you found a number they have in common (21) You do this next
3/7 * 3/3 = 9/21
2/3 * 7/7 = 14/21
Now the next thing you do is subtract your numerators but not the denominators
14-9=5
5/12
you cant simplified so your done! Hope this helps!!!!