Make both denominators the same:
35*28=980
Then change the numerators with regard to the changed denominators
6*28=168
5*35=175
So the new fractions are 168/980 and 175/980. Added together makes 343/980. Simplifying it is:
Dividing by 7: 49/140
Dividing by 7: 7/20
So 6/35+5/28=7/20