Let's take an example to illustrate this case:
positive fraction = 2/7
negative fraction = - 3/5
Now we need to subtract - 3/5 from 2/7
Right?
2/7 - (-3/5) = 2/7 + 3/5
here we need to unify the denominators as follows:
the lowest common factor between 7 and 5 is 35
2/7 = 10/35
3/5 = 21/35
Now back to 2/7 + 3/5:
2/7 + 3/5 = 10/35 + 21/35 = (10+21)/35 = 31/35
That's it
Hope that helps you
